The whole family went together to Yorktown early one morning (8am). The temperature and humidity would begin to make things miserable before noon, and we got there before the visitor center opened. This happened to be the best time to go. Not only was it cooler to walk around, but we had the area free to ourselves with a few locals on their morning walks.
There is a large monument to the battle here, as well as older homes. There still is a Yorktown town that has a shopping area with antique and tourist shops. The shops did not open until 10am or later so we did not get a chance to go in, but window browsing was just as exciting. Plenty of rabbits for the kids to chase in the open grass fields in the area as well.
The main draw to this park is the driving/bicycle tour of the battle grounds. This tour is extensive with multiple road side historical markers. Unfortunately, there isn't much to see other than fields and forest, but it is still interesting from a historical sense. This tour is what you use your flex ticket for, or get vehicle passes. While there is no one checking to make sure you pay for the tour, it is on your honor.