We just got back and had a similar problem with heights (not twins, but siblings) at Blizzard Beach. Our DD is 6 and 49-1/2 inches, DS is 4 (44 inches), and DD is 1-1/2. My oldest DD was too tall for the kiddie area (and quite frankly would have been bored to death there), so DH took her to the pre-teen area which was ok for her. She also did some of the big slides. However, that left me with my 4 yr old and 1 yr old in the kiddie area. My 4 yr old wanted to do the slides in that area and so did my 1 yr old, but I couldn't be at the top of the slide to get them down AND at the bottom to be there to catch them. I really needed another set of hands.
Also, I didn't see them giving anyone a problem that was over 48 inches that were just playing around in the kiddie area, but they were measuring at the top of one of the slides to make sure the kids going down were under 48 inches. But I really think your 7 year olds would be completely bored in the kiddie area. Even my 4 year old was borderline. It really is for little ones. The problem is, you need to be 48 inches to do many of the larger slides.
Overall, we had a good time, but I wouldn't do it again until my kids are older. I think it is a great place if you have enough adults to split the different age group kids but otherwise it can be a little stressful on mom and dad.
We did enjoy the lazy river and the family raft ride, however, my one year old couldn't go up on the chair lift that leads to the family raft ride so we had to hike up the millions of stairs to get to the top (or at least it seemed like a million in the heat of July, lol!). We only did that once. There was a long line for the chair lift anyway.
Hope my rambling helped in some way, lol!