Nebraska

Mahoney is the best local to you for those ages, Indian Cave might be up your alley if you don't mind the drive. I'm not too familiar with out west but I'd be cautious doing that long of a drive with young kids on your first go around. Tips - bring lots of water, chose a more built up site for your first few times to guage your kids, get your park pass and reservation ahead of time, you can pack a lot on top of your roof though you will not pack most of what you pack after your first few times, bug spray is great but sunscreen is necessary, check for ticks when you are done, have some planned and unplanned activities for the kids, pace yourself.