Peakaboo and Spooky Gulches in Escalante are the best candidates. They're slot canyons crossed with playgrounds, and the loop is a good length for little kids. It might be out of your way, and requires a drive down a rough dirt road to the TH.
The Fairyland Loop is Bryce is a great kids hike; lots of great hoodoos, but no slots.
Hidden Canyon in Zion is slotish and really cool. Still have vivid memories from a visit when I was 8.