I had 2400 in fsa, I used for 1 child. Can I use the 3000 dependent care expenses for the 2nd child? Or did I mess up thinking I could use FSA for one child? Their bills are paid separately and I only submitted FSA claims for the 1st child.

Enter all your qualified expenses.  If you paid $2400 of care expenses from an FSA, you can use up to $3600 of care expenses toward the credit.  Just enter both your qualifying children, your care providers, and all your qualifying expenses, and the program will figure it out.

Great, thank you!