Assuming she is the custodial parent, your right to claim the one child is solely based on the divorce decree. Since you released your right to claim that child using the 8332 after the divorce decree, the 8332 would override the divorce decree. If she is amenable to allowing you to claim the one child, she can provide you an 8332 giving you the right.
https://www.irs.gov/pub/irs-pdf/f8332.pdf
