Yes you can. If you are claiming them for 2021 you can get the recovery rebate credit AND the child tax credit on your own return. Congress left a loophole that means their mother does not have to pay back the stimulus she received for them but that you can get it if they are claimed on your 2021 return.
If their mother received the advance child tax credit payments she is required to file a 2021 tax return and enter those payments, though.. Depending on the amount of her 2021 income she may or may not have to pay back the advance CTC payments. Since they are being claimed on your return you can get the full amount for the grandchildren who were under the age of 18 at the end of 2021.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
