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?)
I am working on a similar service for my own needs (and some other friends). But, my current plan is to hit $100/yr or so but the hardware is included in that cost. I am assuming $2/mo for costs of the number and minutes and retail costs for VOIP hotel phones is about $50. Hopefully the hardware costs amortize out and then I can offer a discount to users on subsequent years.
(If you want to go DIY you can save quite a bit of money too- I provided some breadcrumbs for doing that in my comment on the parent)
