That is not uncommon, especially for two earner families with better than average incomes. The withholding tables don't withhold enough. You can minimize that by filing a new W-4 with your employer, claim 0 (as you are doing), and choose "married but withhold at the higher single rate". If that doesn't solve it, there is a place on the W-4 to ask your employer to withhold an additional cash amount each pay period. As you children get older, the problem gets worse. You loose the Child and Dependent Care credit when a child turns 13, and you loose the Child Tax Credit when a child turns 17.
