We went last year and arrived on Labor Day. We didn't see many families with school age kids. We arrived when the parks opened and it was great. The other event at MK which is the Friday and Saturday after Labor day, is the Night of Joy. MK closes by 6 pm on Friday and Saturday unless you buy an event ticket for Night of Joy.
Be prepared if you are staying at the CR, the bands practice their set the night before they play starting at midnight. It makes for a couple of rought nights if you are the contemporary as you can hear the bands from midnight to about 6 in the morning on Thursday and Friday night.