If you are the custodial parent and/or the non custodial parent with a form 8332 then you should amend the 2019 return to claim the child you are entitled to claim. The stimulus payment is another story ... it is too late to get it now based off the 2019 amended return ... instead you will claim the child on the 2020 return to get the stimulus you did not get already.
If this is their year to claim the child then you will need to use the court system to get satisfaction as the IRS will not intercede.
