Fun, we love BB! I think it's the better of the two with kids, in my humble opinion.
I like to get to the water parks at rope drop. Just like the theme parks, those first few hours can be critical in terms of getting a bunch of rides in with less waiting.
You might want to look at a map and figure out a good "home base" for the day to put your stuff. You're probably going to want to find a spot with some shade. I would do this first thing when you get there.
We rented a cabana last summer and it was GREAT, but the price is a bit obnoxious. The cabanas come with bottles of water, several seating options, and an attendant that can bring food/drink all day long (you have to pay for food/drink). Some cabanas are better than others in terms of shade, size, location, structure, etc.
Are you staying on property? Bus or car? Disney has jacked the bus routes to the water parks recently. From many resorts you have to go to a park and transfer, it's such a pain. For example, from Poly you have to take the bus to AK then transfer to a bus from AK to BB.
Your ages are pretty wide. The 3 year old is going to love the kid's area, the others, not so much. Assuming there are two adults, you may need to split up at times.
Sorry for the long post. Feel free to ask more questions. Again, we love BB!