Slow season is January (just after New Years) through the begining of March. Then is gets busy for Spring Break and holidays, usually March through Mid to late April. Begining of May slows down a little bit and then picks up again from the end of May through June, July and August for summer. Towards the ends of August it starts to taper off again and then September, October and begining of November are fairly slow. Mid November through December picks up again for the holidays. The BEST time to come for no crowds is January and February (but it can be kind of cold then) and September and October.
I don't have children, but if I did I would take them out of school for the trip just because it is really busy when the school vacations occur. If you are looking for a slower time to come then you would have to as it is slow when the kids are in school and parents don't want to take them out.