My kids were 4 (dd)and 5 (ds) on last trip - I just don't have that 3rd one in tow! We never hop, at least not when we're there for a full week. It is way possible (and way less expensive) just to leave and go back to the same park - there is plenty to do at each park to make that doable. Just plan meals at the same park you plan to go to that day OR at resorts or DTD if you don't know where you want to be that day.
I think you should definitely do all the parks - if it gets to be too exhausting just plan on doing the mornings at the parks most days - then pool/naps at resort from 1:00 - 4:00 or 5:00 - then a couple of evenings go to nighttime stuff at parks - other nights just go to bed 'on time'. That's what we do and it always works well.
My thoughts on a basic plan:
Here is a sample plan what I would do when my kids were that age:
Arrival day: get settled in - let kids take a nap - either go to a character dinner (like Chef Mickey's - not in a park so no park ticket) or go to a park for dinner and evening show. This plan is only if you arrive at hotel by early afternoon.
Day 1: MK - I know some people say to leave this one for last - but we like to do it once in beginning and once toward the end.
Day 2: AK (I offset our 'all day' parks which are MK and EP with AK and DH which only take us 1/2 to 3/4 of a day) - arrive at rope drop. Not to be missed: safari, boneyard playground, Kali river rapids (if 4 year old is 40 inches tall), nemo the musical, FOTLK, maybe Donald's Safari breakfast. Tough to be a bug might be too scary for the kids - my dd hated it until over 5 years old.
Day 3: EPCOT - Not to be missed: If 4 year old is 40 inches: Soarin, test track, if not: Living with the land, nemo ride, turtle talk with crush, journey into imagination with figment, Acrobats in china, voices of liberty in american pavilion, maelstrom, kidcot stations OR passports that get stamped in each country.
Day 4: Rest day - hang out at pool, maybe schedule late lunch or early dinner at Wilderness Lodge, Hoop De Doo or one of the meals at Grand Floridian so that you can check out some other resorts. Maybe catch MK fireworks from the beach at one of those resorts after dinner.
Day 5: DHS - Toy story mania, muppets, playhouse disney live on stage, voyage of the little mermaid, shrunk the kids playground, High School Musical 3 show and the 'parade' Block Party bash. Maybe think about Play & Dine at Hollywood & Vine meal if kids like JoJo, Goliath and/or Leo and June.
Day 6: MK again - anything you didn't do the first time around (can't miss - and I would recommend in this order if you get there at opening: dumbo, peter pan, Pooh, teacups, Goofy's barnstormer, if 40 inches - fastpass for Splash Mtn. Then later: It's a small world, philharmagic, carpets of aladdin, buzz lightyear, country bears, carousel, plus the parades)
Day 7: last day - maybe do DTD and relax in afternoon then hit EPCOT for an evening meal.
Have fun planning!