While SAB is nice, if your 7yo's are "swimmers", they could think the little kid part (the sand bottom pool, which is truly awesome for little ones) is too babyish, and you'll definitely have to be holding onto your 4yo in the other parts. When mine were both little enough that they both enjoyed the little kid parts, SAB was great. We could sit in the kid pool all day and they were occupied (plus there's a playset right there too). But now they're in-between the kiddie part and the older part (they're 7yo and 5yo and good swimmers), and I'm not crazy about SAB for us anymore. You can't really send them up the slide without walking them over there (it's out of the fenced-in pool area and across the 'boardwalk'), and you can't watch them go down from the top of the slide either. so if you take them over there and go down first, that would work. I think SAB is really beautiful, but it honestly makes me a little nervous with how deep it is. The lazy river part is nice, but deep in parts (if i recall, 8'), and the tubes we used were so high I couldn't even comfortably float in them, I can't see how a child could. Again, it's nice, but you have to really be on top of your small kids in the regular parts.
My favorite pool for a good mix of ages is the boardwalk by far. The clown slide is big and fun enough for any age kid (adults too), but fine for a 4yo too if she's willing (my ds at 3yo loved it). You can watch them go up and down the slide safely, and there's the playset area all in the fenced-in area. It's very contained, which for us, works well. SAB is very big, which some people find appealing, but I really don't. I like the food, bathrooms, slide, pool and playset all in the same area.
I love the poly, except during busy season... the pools are both rather small and get very crowded (we were there easter week though, which makes a difference). At least at SAB, they have wristbands or check room cards or something so it doesn't get overly crowded. At the boardwalk they asked for room cards during the days surrounding july 4th, because of pool hoppers.
And to add, we tried the contemporary for the 1st time last fall, and loved it. The pool is really nice for kids. there are a lot of fountains in the pool, and the slide is pretty good (not as good as SAB or boardwalk, but a bit better than the other deluxe slides). The pool is big enough that it doesn't feel crowded. I just remember one easter at the poly, literally being elbow to elbow in the pool. we had to get out... it was truly ridiculous. other than the crowd factor, it's very nice.
hth!