Huh, I was just thinking about something sort of like this after camping with some friends and our kids this weekend -- we brought FRS walkie-talkies for all of them (cannot recommend this enough!) and on the drive home my four-year-old was asking if he could call his friends on the radio -- rather than getting him a Technician's I was thinking about finding or making some push-to-talk cell/wifi devices for them. It seems like a few of these things exist but they're marketed toward the enterprise (in at least some cases, with a family-style product unfortunately but unsurprisingly being discontinued: https://relaypro.com/families/ ), but it doesn't seem like it would be a hard build aside from making a durable/kid-friendly enclosure for it.
you could try GMRS over FRS which ends up having more range and only costs $35 for a 10 year license (no test / certification needed). I was recently trying both types of handheld walkie talkies and the FRS range was almost 1/5th the range that GMRS was able to do
