I think we need to go even more basic than that. Why is your child not eligible for an SSN? Is your child a US resident? Did you claim your child as a dependent in 2023 and is the box for "Credit for other dependents" checked?
Then, did you actually have an FSA benefit on your W-2 for 2023? With an amount in box 10 of your W-2? Did you include a form 2441 with your tax return, and what does it say?
