That sounds right. Chances are you didn't have enough tax liability to earn the full $600 for yourself, so what you did earn is probably $300 for you and $300 for the 12 year old, as the 18 year old is not elagable for the child credit.
I wouldn't pursue it, since you wouldn't be getting anymore then that. the problem is that the IRS said $600 per person, but that fine print says, $600 only if you made enough money, and not everyone does.


