I think it's more the case that many parents are not tech savvy enough to even know that's possible or how to do it. Also, this create a safety net which seems too fragile, as you just need one family that doesn't do it to potentially expose all their friends.
It's no different to the phones for toddlers at school - they can, they know how, they are just even more affected by FOMO, peer pressure and judgement than children.
The other excuses are all just cope - little Timmy will just find a way around the blocks is true. Little Timmy can also get Heroin if he really wants or just one family might offer all the kids a hit on a crack pipe but it isn't an excuse to keep a needle, pipe and some fresh gear in the living room.
