For such a short trip I'd say not worry about Universal at all.
I'd do 1 park each day, 1 day for the water park, and the 6th day for whichever park you want to return to. Trust me, there's going to be one Disney park where 1 day isn't enough. For us, it's EPCOT - for you, with young kids, it might be MK.
Avoid MK on Saturday and Monday. Sat you have all the locals, and Monday is the first day of most people's trip, so everyone goes to MK first. If I were you, I might do the water park on Saturday: your last day, good way to relax.
If you want to do early entry, plan your park days around that. If you want to avoid early entry (the ee parks are much more crowded) then plan your trip around avoiding early entry days. That's how we do it.
The early entry days are as follows:
Sunday - MK
Monday - AK
Tues - MGM
Wed - EPCOT
Thurs - MK
Fri - AK
Sat - MGM
Also, check on the main Disney site to see when they're doing fireworks and parade at MK; that can help you decide what day to go there. Spectromagic is a real no-misser.
You're smart to not park hop, imho. Park hopping is really ambitious, very tiring, and often takes longer than is worth it. Also, if you decide you're too tired to park hop after planning to do so, you might feel like you're missing something. A park a day is the way to go.

The only real exception I can think of is AK during the day and either Illuminations or MK fireworks/parade in the evening, b/c AK closes so early.
Hope this helps.
