this is one of those 'crazy' situations of the stimulus program.
BOTH stimulus payments were an estimate based on 2018/2019 tax returns, but what REALLY matters is the 2020 tax return. If you are owed stimulus, it will be reflected on the 2020 tax return; if you received too much stimulus, you don't have to return it.
So your ex received the stimulus because he claimed the child on the 2019 tax return (and probably will receive the 2nd stimulus as well) . But I assume you are claiming the child in the 2020 tax return which will create a tax credit on Line 30 (assuming you are eligible - most are).
does that mean that the IRS will be paying out the stimulus twice on the same child and neither parent is required to return the excess? Yup!
