I would check snow conditions wherever you go. The east side of the San Jacintos still has a lot of snow. I have not been up there in a while, but snow if visible on the east facing slopes, from my house. I would imagine that upper elevations from Idylwild still have a lot of snow. Also the Devil's Slide Trail might be a little tough for a kid.
Another option might be the Cedar Springs Trail from Garner Valley. The trail from the Morris Ranch Road Trailhead to Cedar Springs Campground is about 4 miles one way. A little steep, for about 2 miles, but easy trails (switchbacks). Elevation gain is about 800 feet. The campground is in a grove of incense cedars with a year round spring.This is Desert Divide country with great views. I don't know if there is any snow, but probably not.
Probably won't see many hikers unless the PCT through hikers are hitting the area.
Here are some pictures of the hike.