Please read this thread on the old forum:
forumsarchive.xbox.com/.../ShowPost.aspx
I have finally received a fix for this issue after 5 months of chasing MS for it. It is very simple to do. Put all accounts associated with the Family Pack on the same console. Sign into the console as the "adult" account and go to the "Family Settings" heading on "My Xbox" section. Once in "Family Settings" you should see any "child" accounts listed there. There should be a menu option to remove the account from the family. Doing this will set the account back to the proper setting. It will state something about having to associate it with a new Live ID but it's a bogus message.
Once the account is removed, sign out of the "adult" account and then "Recover" the former child account to the console. You will have to agree to the terms again but once recovered it should be removed from any family and no longer be a child account! At least it worked for me....