As long as she marked the box that she can be claimed as a dependent by someone else and you did claim her, but you did not get the full payment for her, then you can claim the difference on next year's income tax return in the section for Recovery Rebate Credit. They may also look on next year's return to be sure you claim her on that return to include it in your next year refund.
There is no reason for either of you to amend your returns. It sounds like both of them were filed correctly. IRS may later send you the difference but it is unknown at this time if they will or not. The 2020 Recovery Rebate credit determined whether you are entitled to an additional payment for the 2nd check. IRS was supposed to calculate the amount to be sent to you for the 3rd stimulus check and send it to you.
