And for the other half of the equation, please remember that the IRS does not care what the Divorce Decree says about who claims the child, and does not follow that.

I use this tool to determine who the IRS says is entitled to the Dependent

https://www.irs.gov/help/ita/whom-may-i-claim-as-a-dependent

If the answer from that tool is not who the decree says gets to claim that year, Then use a form 8332  to allow the other parent the dependent for that year.