As
@BamaBoy said, the hike is as long as you make it. It is out and back, so at whatever point you turn around is up to you. Definitely saw plenty of young kids doing the hike. It will depend on the water flow how hard the hike is. We did in early June and just the week before it was closed because the water was flowing too fast. The deepest it got on me was probably high thighs, I did fall at one point, so I did get pretty wet. Make sure phones are in waterproof bags. We rented the gear at the park entrance but I would say maybe half the people had rented gear and the other half just went in their own shoes. I personally was happy for the pole with the rented gear. You definitely want to spend the night at Zion, 2 nights would be best, and not try to make it a day trip from Vegas.