Unfortunately, if you can't pull your kids out of school, you'll be limited to spring break, summer, and Christmastime, which are typically busy times of the year at WDW. Spring break is extremely busy, and Christimastime is the absolute busiest time of the whole year. Spring break might be an option, since school breaks seem to be scattered over about a six-week period, with the busiest times usally being the week before Easter and the week after Easter. That tends to make the parks very busy, but not unbearable.
I would avoid late December like the plague! All families across the US have the same two weeks off, and it's horrendously crowded.
That would leave early summer. When do your kids get out of school? Luckily, the starting and stopping times for schools vary greatly throughout the country. Some schools start in early August, and get out in early May - a great time to go to WDW! If that's an option for you, I'd definitely recommend it.
Our schools in the Northwest don't start until the day oafter Labor Day, so we end the school year pretty late - mid June. That would still be a good time to go if you can.
I would avoid July if you can - there are lots of tour groups and it tends to really impact the parks and some of the hotels.
You might try going on a web site like Easy WDW (
www.easywdw.com). They have lots of great info about the pluses and minuses of the different times of the year, and even have a crowd calendar predictor.
Whatever you choose, I hope you have fun!