The stimulus credit/rebate is only based on what you claim on your 2020 tax return. Amending your 2019 return won't change that. You might qualify for up to a $2000 child tax credit, depending on your income and other credits. You might also qualify for a larger EIC or larger dependent care credit. However, if the reason you did not claim the child is that they did not have a social security number, the most you would get back is the $500 credit for "other dependent."
Why didn't you claim your child before? That might be important.
