There is no requirement for you to return the stimulus payment for the third child, even though her grandfather received it. This applies to families where parents are divorced, as well. Both households may receive the payment.
Click the link for the details on how each stimulus payment is calculated:
Stimulus payment 1 calculation
Stimulus payment 2 calculation
For more information, see Economic Impact Payment Information Center — Topic J: Reconciling on Your 2020 Tax Return
