We went over Memorial Day last year-- 9 days with the weekend right in the middle of our trip. There was definitely a difference in crowds from earlier in the week, but it wasn't too awful. (We just decided last night that we're going to plan another trip for that same week!)
I'm not sure how old your kids are, but we absolutely needed the park hoppers. My kids were 9, 4, and almost 2. We usually got up and went to EMH park until 11-12. We went back to the hotel to nap (yes, we ALL took a nap) and then went back to a different park from about 5-8:30 or so. For us, it was extremely hot in the afternoon, so I don't think we could have stayed in the park all day without being miserable even if we didn't want to nap. We did EMH at MK 3 times during our trip and honestly got more done than we would have the whole day if we had gone later in the afternoon. We did every ride on our must-do list with almost no wait, but many of these rides had 70-90 minute waits in the afternoon.
For us, the park hoppers are definitely worth it. We went to two parks every day of our trip. We went to AK one day and it was so packed we had to leave. We would have been really disappointed if we just had to sit at the hotel the rest of the day. Also my kids loved taking the monorail from MK to Epcot. One day we walked into MK just to get their haircut & confettied, got on the monorail, and went over to Epcot. We didn't, however, make any ADRs, so we were really open to go wherever we wanted. If we had all of our days planned out maybe we wouldn't have used them as much. I've seen many people say that they do not need/use them. You can always wait to add them on when you get there.