I second Watson Lake in the Prescott area, my kids always liked that one. Same with Horton Creek, they might only make it halfway, but they’ll have fun in the water.
A few other ideas for kids that age (my kids did all of these around that same age):
Walk the perimeter of Bear Canyon Lake. It’s pretty easy, just a few miles. Water and birds for the kids to see. A few spots where you might want to hold the younger ones hands, but my kids did great.
Red Mountain trail north of Flagstaff. No water but the kids may like exploring in all the rocks and little pathways at the end. After you’re done with that, you can stop by the Slate Lake lava tube just a couple of miles away. It’s much smaller than the popular one by Flag, just a few hundred feet, but you’ll likely have it all to yourself, and it’s easier for younger kids.
Marshall Gulch by Mt Lemmon is great for kids. Get there early, the parking fills up. There’s a cute bakery/fudge shop in Summerhaven the kids may like. Some of the other trails at the top are pretty kid friendly too.
When it cools off around Phx: maybe Marcus Landslide trail in the McDowells. Short and easy, with fun rocks.
More of a late winter, after some good rains, Arnett Creek by Picketpost is fun for the kids.