Unless they are animal lovers, they will like MK the best. Hit Fantasyland and Tomorrowland for sure (skip Stitch unless they love him).
MGM is for an older crowd, certainly older than 4 & 6, thou if you get there they will enjoy Mermaid, Muppets and some of the shows (you can decide which of the 4 are best by the titles).
Epcot rides are more severe, but they'll enjoy Test Track and the hands on displays in the pavilions around Future World. World Showcase is basically shopping and eating.
Animal Kingdom is great if you kids like animals. We like it, but many find it a bore.
As for Universal, stay on property to get the unlimited priority ride pass - how nice it is to walk onto EVERY ride with NO wait

- and to be able to walk to the parks in 10-15 minutes max

! No need for buses or ferries or monorails. So convenient. Visit Islands of Adventure first. They will LOVE that. Water rides, very cool Spiderman ride, roller coasters, Olive's boat to play in, kid's sections with smaller rides. Very fun park. Universal Studios is fun, too, but more for your age boys in IOA.
With a 7 day visit, I'd get a 2-3 day MK park hopper and plan on MK the majority of the time and the other parks for partial days, allow for 1-2 days of rest interspersed with your MK days, get a 2-3 day Universal park hopper. Universal is easier to traverse and just as much fun without as much exhaustion in travel time.

The Universal Royal Pacific Resort has a great pool which includes a play ship with squirt guns, etc. They might really enjoy that. Hark Rock has a water slide, but not sure if it is better suited for young boys.
Have fun planning!
