In the age of FT audio calls and such, it seems out of place. I get paying for the hardware, and even a bit of a premium because the UI has so many parental controls. But charging a monthly fee seems surprising since the differentiation versus FT/GV/etc. is on the UI side, not the "you can talk to your friends" side. There is no monthly cost to a UI.
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?)
FT audio calls are in the same boat where if everyone you knows has an iPad you can make calls within that walled garden for free and don't need to be allocated a phone number. Admittedly more useful since so many people have an Apple product.
Anything where you want a phone number and to communicate with the rest of the phone network, I'd expect to pay money for it. Google Voice is the (only?) oddball here.