It depends on your kids, their ages and the level of activity they are used to. Generally I would say the Moses Spring to Rim Trail loop. I've seen plenty of kids having a nice break at the reservoir. Even with the caves closed for COVID it goes through an open cave area and up a big staircase that's pretty neat. Very fun for kids and adults. Note, if you get there too late to park at the trailhead, the hike to the trailhead can add a substantial amount of time. It also gets VERY hot in the summer so be prepared. I wouldn't recommend this park to people with very small children who are not used to hiking a lot. If you aren't the kind of person who already owns one of those backpacks you put your toddler in, I would not suggest bringing a toddler. Most trails at this park are pretty steep and rugged. Last time I was there, I saw someone trying to roll a stroller to the trailhead. There are NO stroller-friendly paths in this park.
The Balconies Cave is fun (when open). You will need a flashlight, but it would be fun for older kids because there is climbing and crawling. The Bear Gulch trail out to the reservoir is also a fun one. There is a "cave" (more like an overhead arch, no crawling or flashlight needed). The reservoir is a place to find birds, lizards, and sometimes frogs.
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