I made something using a VOIP phone and voip.ms. What provider has a $5/mo kids cell subscription!?
voip.ms is about $1.30/mo with the cost of the phone number (DID) and "minutes".
I have some more breadcrumbs on this thread if you are interested in details on my setup too.
Regarding minutes, how many minutes? My kids basically hog my VoIP "landline" talking to their cousins 50% of the time they're home. (They're actually playing games together and using the phone as voice chat). I've been thinking of trying to get a second line for them to hog but didn't want anything with limited minutes.
My cellphone provider offers a limited kids cell subscription. Things like "can only call 5 different pre-approved numbers, no data traffic". I think the business model is that after a period of time, people bump into the limitations and upgrade to the $10/mo version.