Best (for older kids) would be a dumb cell phone like we had in the 2000s. Good for phone calls, texting, and simple offline apps like casual games, camera and music player. Maybe email. Definitely no web browser, youtube, or social media crap.

I don't know the extent to which such devices are still manufactured today.

There are: https://www.hmd.com/en_int/nokia-3210

(and it's not the only one, also check KaiOS phones)