Agree (as usual) with Hoops and the others. For I-Day, you really need very agile, active grandparents who have no mobility issues and who can handle many hours in the hot sun or pouring rain. If they meet that criteria, then it depends on your DD and your grandparents. There isn't a lot of interaction between the plebe and family on I-Day. It's a very quick good-by hug in the morning sometime between 0530 and 800, then independent activities for parents like Supe's brief, picnic, and MidStore shopping. The Oath ceremony is about 30 minutes around 5:30 pm and most have trouble picking out their kid. Then about 25 minutes to chat and say your good-byes. That's it.
PPW allows much more time with family. What I suggest is to have the grandparents come for one of the three days. That way, they can see the Yard and what your DD has accomplished. BUT, it prevents them from getting bored if your DD just wants to relax and/or prevents your DD from thinking she has to "entertain" them all WE. There will be many more opportunities during the course of the four years for them to participate in events -- depending of course on where they live. There are football games, other sporting events, parades, Commissioning week activities such as Herndon, performances (theater, choir), etc. that are generally open to the public. Or they can just come for a WE at some point at some point.