I saw a lot of kids wearing wristbands. Can't find a mention of why on the web site. What are those for?

I believe they are for the people with a day pass so they can go back and forth to their car that day which would be parked outside of the park. The paid parking is inside the park.

You get a wristband when you pay. Let allows you to enter and exit the park all day you choose to

James is right. Most of the time they are for adults so they can back and forth to the car and back, if a kid wants one they give them one aswell just so they don't feel left out.