I hope you see this response ---
If the kids are just starting on Mountain Biking, I'd start here:
Spring Mill Indiana State Park!!
They have a "Dirt Jump Park", basically a smaller skill-practice mountain biking course where they can get used to bumps, turns, uneven services, inclines, downhills, etc. We took our kids here a few years ago and learned we need to wait longer before going on a real mountain biking trail where experienced adults could be flying past us.
The park also has a pioneer village and a working mill where you can watch & buy cornmeal ground right there. The cornmeal was AWESOME. I need to get back there. We didn't even see everything when we were there. But that was a really great vacation all the way around.
But if you are just starting to explore mountain biking w/ your kids, I highly recommend this as a place to start!