That is what we are thinking. We are withdrawing 529 funds as we use them to pay for university.
We did this also. One daughter graduated right on budget. The other graduated way under budget. We are using the left over to help to pay for a graduate program.
We did this too, except in the other direction. It is quite easy to transfer 529 funds from one sibling to the other.
Strictly speaking I am pretty sure that it is also okay to wait for a grandchild. However, this could take quite a while in many cases, and might not happen.