I would go out on a long limb and say that if you enter the amount of payments they actually received for the year and enter the dependent information in properly, the software is going to run with it from there.

"but are now eligible because of their 2020 tax return?"

See if this clarification helps you understand it better, because that isn't what happened.

They got Advanced Funds against a Projection of what 2020 would look like. But the advances are based on 2019 or 2018 tax forms. The 2020 tax info is the Actuals info used to reconcile what you are qualified to get against what you already got paid to you.