Last I checked, (which was http://www.google.com/support/accounts/bin/answer.py?hl=en&#... right now), if you provide your credit number and are willing to pay thirty cents, you can re-activate the account. They specifically say that it can be done by parents/legal guardians. Problem solved, 'nuff said.
We had the same situation. My 11 year old daughter has had a Gmail account for many years and started using a couple years ago. Everything was fine until she tried to accept a Google+ invite and was locked out. Unfortunately this was more than just losing a few silly emails as this is her primary account for lots of school activities.
My wife found the credit card verification link and got the account reinstated. I wish Google+ would make it more clear or just disallow underaged accounts and not invalidate the whole Google account.
My daughter had hers disabled, never found the reactivate link. It sucks because she is now 13 and a non-issue, but lost her email address of 3-4 years.
