If you're looking for some honest advice --- I think you've scheduled WAY WAY WAY too much... especially for a first-time visitor.
Walt Disney World is WAY larger than any first time visitor could imagine. For example, it can easily take an HOUR to park your car in the Magic Kingdom lot, buy tickets, take the ferry or monorail to the park, go through security, and get your bearings. (The same goes for driving to Universal or Sea World)
All of the parks are HUGE. A good majority of your day is spent just WALKING from attraction to attraction.
Also, it's summer here in Orlando... and it is hot, humid, and MISERABLE. The crowds are HUGE. Unless you pace yourself (in other words, "slow down"), you're gonna burn out by mid-day from the heat and the crowds.
I'm assuming you're staying ON Disney property. If you're not-- based on your schedule-- I think you're in for additional frustration.
Unless you've got amazing stamina and energy, I do NOT recommend doing 5 straight days of theme parks. You and your kids run the risk of getting grumpy, and yelling at each other instead of having fun.
I think a first-time visitor can do Epcot and MGM each in one day (get FastPasses for Rock-n-Roller Coaster and Soarin' AS SOON AS YOU GET THERE!). Universal and Sea World can also be one-day trips.
However--- it is IMPOSSIBLE to do the Magic Kingdom in one day-- especially in the summer. And with a 6-and-8-year old... I think most would agree... this is the park your family will likely enjoy the most. (I also think Universal would be the LEAST favorite. It's a great park, but the attractions are not very kid-oriented.)
If I were you, I would take ONE day off in the middle of the week. Go to the pool, visit a restaurant, or do one of the many recreational activities at Disney--- like renting a small speedboat, playing mini golf, exploring a different hotel, catching a dinner show, or even a water park. More importantly, SLEEP LATE. You'll thank me later!
My recommendation if I were in your shoes--- Do Magic Kingdom on Day 1, MGM and Epcot on Days 2 and 3, Rest/Pool on Day 4, and Magic Kingdom on Day 5 (or, if you felt like you saw enough on Day 1, go to Sea World)
I know its probably disappointing to hear my advice. You want to do it ALL. But trust me, you'll have more FUN if you take it slow.
It's kinda like an all-you-can-eat buffet. You can try to cram it all in, but you leave feeling sick. Or you can savor a handfull of things... and try others on your next visit!