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.
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.