Here's an idea: get an old phone supported by lineageos and just don't install any apps or even f-droid. This is essentially a modern iphone 1(the first). Not totally dumb but dumb enough to not be a distraction.
I recently got a small basked a little larger than my phone. I put my phone in it when I get home from work. It's a good example for my kids and it helps me to actually be present with my kids instead of being on my phone half of the time.