Hi Regina!
That's a tough question, only because kids (and parents) generally take off for Disney during those breaks. Summer is BUSY (peak period), the same can be said for Christmas break. Some kids actually start school a little after the new year (say, January 6th or 7th). If that applies for your children, you could look to visit around then. Also, depending on when their Spring Break hits, you may get less of a crowd if it's during a lull where not as many children are out.
The best approach, if avoiding crowds is the goal, is to look for any off-season break where your children get a long weekend. Off-season is generally September-March, excluding major holidays. Perhaps a shorter vacation during an off-season period may give better returns in terms of crowd levels.
But Disney now is never really slow, because a number of international families are traveling during periods when US children are back in school.
So, hopefully you'll find a good time to bring them. Early November, February, and early March have always been better periods for our family historically, in terms of lower crowd levels.
Good Luck!