If you want to maximize your recovery rebate, you should claim both of your children this year. This of course is assuming that the children lived with you at least half the year (such as because you and your partner live together). This is perfectly legal and a result of the way the credit is written into the tax law. The recovery rebate is based on how you file your tax return now. If your girlfriend claimed your child last year, and the IRS use that information to make an advance payment, but you claim the child now, you will get a current payment and your girlfriend will not have to repay the advance payment. If your girlfriend were to claim the newborn but not the four-year-old, your girlfriend would not receive any additional money for the newborn, because the IRS calculates the credit based on the number of dependents and not who they are. If your girlfriend claimed both children on her tax return, she would get a stimulus payment for one additional child, but if you claim both children on your tax return, you would get an additional credit for both children.
