It's interesting to see that so many people have good input on this topic.
For some "out of the [Obi] box" thinking (sorry, I just couldn't resist): forget about Google Voice. The limitation Robert described is one reason to consider an alternative: Buy each kid a OBi 200, with the advantage previously mentioned that it becomes "their" telephony device that can be moved or changed with ease. Attach a DECT 6.0 cordless phone to each one -- I think DECT is sufficiently robust to support multiple base station radios in one building. This eliminates any wiring issues. Provision each OBi with a SIP ITSP DID (inbound phone number) and a calling plan, from a good service provider like Callcentric or
voip.ms. Eventually, the kids can contribute towards, or fully pay for their phone service. If "parental oversight" is desired, you'll have a detailed call log for each user. The numbers will be easily portable, they'll support E911 emergency calling, and they'll have all the features of modern phone service, like call waiting, call forwarding, caller ID with name, 3-way calling, voicemail, etc.