[Moderator edit] The only way it could harm the OP's grandparents would be if it would cause a hit to their credit score and this would for some reason affect their ability to receive credit for other purposes. That doesn't seem to be the case here. Almost all grandparents want to help out their children and grandchildren, and here's a way to do so at no cost to them. It doesn't hurt the issuing bank either, at least not any more than all the credit apps many do all the time with no intention of using the card beyond whatever spend minimum may apply.

I could add that I opened a credit card account for my elderly mother a year ago, with the intent that she could use the miles to visit family in another part of the country, a trip she was making periodically for cash. Now she has suffered health reversals and almost certainly will never fly. I do plan to use the miles myself, and don't see anything wrong with it.