Yes, whoever claims the child (age 16 and under) on the Tax Year 2020 return will get any outstanding stimulus payments. If the child turned 17 that could be why she would not have received a round two child payment. Here is the way the program is set up right now and is of course subject to change at anytime.
If your ex-spouse claims the child (age 16 and under) on her 2020 return she would get a credit on her taxes for a child round two stimulus payment as she would report that she received the round one payment already.
Keep in mind if you claim the child (age 16 and under) on your 2020 return, and assuming you did not receive any stimulus payments for the child to date, will indicate that on your return. So assuming you meet the income requirements, you should get a credit on your taxes for child stimulus payments rounds one and two.
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