Came here twice with my 7 & 4 year olds and did the gateway loop. Though this 3.6 mile loop was fairly easy for kids, 4 year old had to be carried 1/2 the time since he didn't' have much endurance. This loop is pretty safe & wide but some parts are pretty rocky. I did preferred going counter clockwise on this loop b/c coming down is less steep for the kids.
Pros -
*Easy parking
*Clean trail w/ beautiful desert views
*Clean and modern restrooms
*Volunteers are there to answer you questions (I was there on Sundays)
*Informative maps & diagram to show the level of the trails
*Doggie water fountain
*Some cell phone reception (Gateway loop)
Cons -
*Busy on weekends mornings
*Watch out for mountain bikers behind you