If you want to call POTS numbers, seems reasonable to pay a fee. voip.ms (of Canada, I think) charges ~ $2/month for a number and ~ $0.01 / minute for calls to US/Canada, calls to other subscribers are free. They work with deposits and a balance. Something in that range seems reasonable... maybe a little bit more if there's some work to make it specifically kid friendly (parental controls for incoming/outgoing calls?)