Hi, Dina! I have three kids also, and especially when we're on vacation, these girls can eat!
For my gang, I find either the buffet breakfasts or the pre-plated, all you care to eat breakfasts are the best way to feed a starving brood without breaking the bank. Chef Mickey's is a huge favorite, and there are so many breakfast options there, no one can complain about not finding plenty to eat. A big benefit to Chef Mickey's is that you can get an early morning advanced dining reservation, and if you use Walt Disney World transportation, you are just a short monorail ride away from the Magic Kingdom or EPCOT.
Another great - and theme park convenient - location is the breakfast at 'Ohana. This is not a buffet, but they will bring you as much as you'd like of anything. You'd be amazed how many Mickey waffles three girls can go through!
If you are planning a day in the Magic Kingdom, you can have breakfast with Winnie the Pooh and friends at the Crystal Palace buffet. Plenty to eat, great variety, and the best part is that it's inside the Magic Kingdom, so once you have eaten breakfast, you are already inside the theme park!
Most days, most of the theme parks open around 9 AM. I'd allow yourself an hour for breakfast, and at least a half hour to travel from the restaurant to the park. If you want to be there right when they open, you may want to get your reservations for right around 7:30.
Enjoy your upcoming vacation!