Your Grandfather can pay for any expenses that he wants to. He can't get a tax deduction for the tuition and fees unless he is claiming your children as a deduction, however. Only the person claiming them as a dependent can claim the tax deduction.
As your children's custodial parent you are the person(s) authorized to take a tax deduction for your children for every year until they are 24 as long as they are full time students. If you want your grandfather to be able to claim them as a deduction then you need to not claim them in any years that he is paying their tuition and expenses.
[Edited robertb1326 01/19/2022]
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
