There is no shared custody or 50/50 custody in the tax law. Whichever parent the children lived with for more nights during the year is the custodial parent for the entire year. If the children are only with you for the summer, and they are with your ex the rest of the year, then as far as taxes are concerned your ex is the custodial parent. That means that you cannot claim the child and dependent care credit at all. And as I said earlier, it makes no difference which parent claims which child as a dependent.
If you are not the custodial parent, you need a Form 8332 signed by the custodial parent in order to claim one of the children as a dependent. But that still does not allow you to claim the child and dependent care credit.
On the other hand, if you are the custodial parent you can claim the child and dependent care credit for expenses for both children during the time they are with you, even if you only claim one of them as a dependent, provided that you meet the other requirements for the credit.
