> Vendors shouldn't sell unlocked devices to kids.

This part is neither necessary nor sufficient.

Put aside the Orwellian premise of "devices are locked by default". People keep making the analogy to things like cigarettes, but if a kid wants a steady supply of cigarettes then they need a steady supplier. If they want an "unlocked device" they just need money and Craigslist, once. It doesn't matter what you make Walmart do and it correspondingly doesn't make any sense to involve them.

If your kids have enough unsupervised money to buy electronics then you're either fine with them being unsupervised or you already have bigger problems than a used laptop.