@Nataleigh7
Even though your ex-husband passed in 2021, someone had to file a 2021 tax return for him. (Maybe his mother filed it?) Based on your agreement to each claim one child, your ex was still entitled to claim the one son on his final tax return for 2021, and to get the advance payments and the child tax credit for the one son that he claimed. The CTC was not lost. It was legitimately claimed on your ex's final tax return. So I don't think you can get the CTC for 2021.
For 2022, no tax return will be filed for your ex, so you can claim both children for 2022.
If your divorce decree says anything specifically about claiming the children in the year that one parent passes, that might change the picture. You could check with your divorce lawyer.
