That is unclear. The stimulus payments were never paid out to dependents, they were only paid out to the primary taxpayer, with the dollar amount being determined, in part, by the number of dependents each taxpayer claimed. You would have to file amended tax returns for 2020 and 2021, to include your child as a new dependent. The IRS will review the claim, and if you are entitled to claim your child as a dependent, and if no one else claimed your child as a dependent in the same year, and if adding the dependent, increases the amount of the stimulus rebate, then you may get a rebate. (Also note that in order to qualify, your child must have been a US citizen or a resident of the US, and must’ve had a valid Social Security number before the filing deadline for the 2020 tax return, which was July 15, 2021.)
if your child was claimed by another taxpayer in those years, then it is likely that taxpayer got a credit or rebate for the child, even if they didn’t remember it. And if the child was previously claimed as a dependent, then for you to file an amended return, and claim them as a dependent, will cause the IRS to run an investigation to find out which person who claimed the child was actually allowed to claim the child
