Get a key

Definitely make sure you think through all the physical security implications of having your house automatically unlock in any scenario.

Have the house auto unlock when getting home on a bicycle, sounds convenient until, as you point out, they could get stolen and now the thief has a convenient way to unlock your house. So you would not want that.

You would definitely not want the house to STAY unlocked when something like a tag is in range. If your kid is home alone, you want them to be able to re-lock the house (or in general, you want to be able to lock your house while the kid is home).

Whatever solution you wind up with, you are going to be trading physical security for ease of use (and complicated fun task). Be safe. Make sure the tradeoffs are actually thought through and worth it.

Security wise, while I love automating everything, I personally would just give them a physical key to the front door. (Or an RFID keyfob system).
What you'd be achieving is the equivalent of keyless car entry, with the additional downside that your son can't choose not to open the door if something sketchy happens.
And instead of entrusting them with a traditional key that they can treat responsibly, you're just putting something in their backpack that they don't have to think about.

If you really want to do it, basically anything in homeassistant that has wireless capability and a state would probably work.
A zigbee radio, and pretty much any device doing anything would do it.
When device_name becomes available, activate door opening.

So they are old enough to be home alone, but not have phone access in their home? What's the plan for if they need to call out of the house? Go somewhere else and borrow a rando's phone?

Either an ESP32 BLE proxy via MQTT with a Tile-like beacon in the backpack, or a cipher lock with a Kid Code are probably the most straightforward solutions.

If you have Frigate NVR set up, there's a project called Double Take that does facial recognition. Looks like a bit of work and needs training, but it could be a fun project.

Are you comfortable with flashing microcontrollers? https://esphome.io/components/bluetooth_proxy.html might be a solution to your BLE beacon idea. buy any esp32 board for like 5$, put it somewhere in your living room or wherever makes sense and have it proxy BLE scans to HomeAssistant.

Maybe you can go a bit lower tech. There are "smart" door locks with a keypad that have Bluetooth, but are not otherwise cloud enabled. Then find a small device you can use as a Bluetooth proxy and position it near the front door, to keep tabs on it. Our lock is from Kwikset, and while there is an app to program it, once it is programmed it is pretty much stand-alone and won't need the app anymore.

The lock does have Home Assistant integration but I haven't tried it yet because I haven't done the Bluetooth Proxy yet.

Have you considered an implantable NFC chip in his hand?

Honestly I'd just go low-ish tech and give them a key, or install a lock with a keypad and teach them a number. The security problems with automatic smart locks are too big, in my opinion.

I've been thinking about exactly the same problem.

We want to give our near-10yo daughter her first phone, but she's not allowed to have it at school. She's also getting to the point where she can be trusted at home for an hour or so before one of us gets home from work, so I also need a presence detection method that doesn't use a mobile phone.

My best theoretical solutions are like those already suggested here: an ESP32 BT proxy detecting a homebrew BLE beacon in her school bag, or detect activity on her iPad/the TV. But neither of those are reliable for all scenarios - she obviously doesn't take her school bag to her friend's house, and doesn't always use her iPad or the TV.

The only other thing I'm pondering is if I could setup facial recognition using our video doorbell. I use Frigate with a Coral TPU, so hoping there's a project out there that could possibly do that.

Latchkey kids of the 21century have it worse than the originals, parents not there for them but now they're also helicopter parents spying on them and controlling their actions remotely. I guess we should call them drone parents.

A fitness band will have low power Bluetooth you can detect. You can get a miband for $20-30. You can detect it with one or two $5 esp32 boards running espresense.

Just get an Apple Airtag and have him carry it around in his pocket (just make sure he doesn't leave in there it so it doesn't accidentally get washed). If you need to see if he's at home just check it's location with your phone. Place it on a keyring with a house key.

Perhaps don't do it directly. But have the system assume he is home based on various things. Motion sensors, media playing, lights on or changing states. Things like that.

What I've done is wired a different type of USB port to each door where the doorknob would exist. My kids and spouse all carry a different thumb drive for each door with the proper conversion adapter permanently attached so they know which door it belongs to. Each family member only uses the door for which they have the "key". My wife prefers the back door. The USB port solution helps to deny access to certain rooms inside the house too as I've wired ports on those doors too. For instance I have a room where I keep the liquor that I can block the kids from accessing.

Each port is connected to a different RPI5 that runs software that upon a device insert reads a special file named for the date. Inside that file is the name of the family member so the script knows who it is. It then updates a spreadsheet hosted in Google Cloud so I can view it from anywhere. Google sends me push notifitations when this happens.

The beauty is that if one of my family loses their thumb drive I just make another and rotate the adapters so the old one won't work anymore. My wife is the biggest culprit of this. She doesn't work and is home all day but she still has a pretty busy life based on all the action that back door sees. But it has lead to a bug - more on that later.

It's a pretty flawless solution - I'm so proud of it I've given my tech savvy neighbor a thumb drive of his own to help test the system. Like I mentioned I have one bug to fix tho.

It seems like every time my wife loses her thumb drive there is some ghosting in the logs. For instance, I'll see a push notification with her name and a the back door opening and then a few minutes later another rear entry with her name attached but never an exit between events. Maybe an hour later then I'll see an exit.

My neighbor always seems to do his testing while I'm at work too cuz I'll see his name show up. My wife has mentioned he'll stop by for a half hour or hour a couple times a week and answer questions about the system and tech. I think she wants to get a job in tech.

Another bug I've only seen happen a few times is the back door opening with my wife's thumb key, the neighbor entering the back door, and then my wife's name showing another back door event. I've called her on the phone to have her (and the neighbor if he's there) troubleshoot during this and she is definitely home but she sounds really hurried and out of breath. I don't know what she is watching on TV but it sounds like several guys arguing and when I ask why it's so loud she gets worked up and has to mute the phone for a bit.

Actually now that I think about it I've started getting notifications of back door entries after my wife has gone to bed and I'm working on projects in the basement. It's soundproofed down there so I can listen to music without waking her. I swear tho I'll hear some off-timed thumping after these events.

But yeah, I really like the tech I got going on and my wife thinks it's the best idea I've had. I think for v2 I'd like to implement a secret knock each family member has instead of the thumb drive so there's nothing to lose. It would be much easier for my wife's back door entry. I swear she squealed with delight when I told her how much time would be saved on entry just by a few properly timed forceful hits to a sensor on the back door. She wanted to try it out immediately!

Good luck!

Disclaimer I don't know the ins and outs of HA nor have I used it.

I'm not sure if you have the automatic lock/unlock part figured out but that's the part I can't help with.

In terms of detecting when kid comes home, some ideas:

A low power esp32 board coded to connect to your home network, then if the device name or MAC address is discovered on the network, or have the device send a message every 10 seconds or when a button is pressed.

NFC card and reader in front of the door, can be "hard wired" rather than over wifi if you prefer. The card doesn't run out of battery but access still can be assigned or revoked if lost.

A passcode: many hardware store doorlocks offer this, if the main problem is your kid losing keys. Then just do automation inside the house as you see fit from motion detectors or whatever your heart desires.

E: a few people ITT don't like to have fun thinking of ideas how to use home automation, even if it's unnecessary.