There is one thing that you might want to consider. I don't know who else you are traveling with, but I gathered that your grandparents are taking you there, right? Have they ever experienced Universal before or know what it offers?
This is just something to consider, and I hate to make generalizations since I obviously don't know anything about your grandparents, but using my own as a reference, 5 days at UO would leave my grandparents pretty bored with little to do. There was very little at either parks that they wanted/could ride, especially at IOA when they went for one day to both parks. Citywalk left very little for them either as there isn't much aside from restaurants and nightclubs, so I couldn't imagine what my grandparents would do for even three days there. Are your grandparents the same way in terms of not wanting to ride thrill rides? If so, it is definetely something to consider....you could possibly keep yourself entertained for five full days, but they might not. If they like or don't mind thrill rides, nothing to worry about.
After the parks close though, all there is to do is Citywalk. If you and your family aren't big night-life/club-hoppers, there isn't much else there besides restaurants and a movie theater. There are very few shops to wander through and depending what time of year you go and how late the parks are open, most of Citywalk could be easily explored in the evening over three days.
Again, it does depend on the crowds, your ability to use express passes, the speed at which you explore the park and whether you are staying on-site or not for front-of-the-line access, but 3 days are probably more than enough, even during the really crowded season.
Is there any other specific concern any of us could answer in terms of what there is to do at UO, besides if there is enough to do over 5 days? (To which my answer would be, maybe, if you don't mind repeating what you've seen a lot)
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