Magic Kingdom is so big and with your kids being those ages, I'd say you'll never get it all done in one day. You will need at least 2 days, if not 3.
It takes us a good 3 days if we want to see and do all our favorites.
One land can take a half a day even starting at park opening.
You have to take into consideration, walking, lines, resting and eating.
We always start in Fantasyland since that is our DDs favorite. If we are there at park opening then we can finish Fantasyland by 12:00p. Then we usually head to ToonTown. That can take the least amount of time for us because all we really do is Mickey and Minnie's houses and Goofy's Rollercoaster. Then we can take the train to Frontierland and do some grown up rides like Splash and Thunder but if it's too crowded and the lines are too long even with FAstpass, we usually run over to do Alladin and then leave.
Sometimes after Toontown, we use the secret back walkway and head to Tomorrowland instead and do Space Mountain (fastpass if we have to) and Buzz ride. Tomorrowland doesn't take too long for us because all we really do is Space and Buzz...sometime Astro if the line isn't long. My kids didn't like Stitch ride so we don't have to take that.
But we usually leave after Fantasyland/Toontown and do the other lands another day.
The lands that take the most time for MY family are Fantasyland and Frontierland/Adventureland, then Tomorrowland/Toontown.
Good luck with your planning!