Over the years, I have camped at Mt Pinos CG on, surprise, Mt Pinos, and it sits at the upper trailhead of what we call the Jump Trail. I can't remember if it is really named Harvest or Southridge trail. In any case, it is a fun and pretty short trail that is not too hard to ride back up from the bottom of the trail, either on pavement or back on the trail.
EDIT: When my son was much younger and had a one speed kids bike, he would ride down the trail and then I would hook a rope and bungie cord combo and tow him back up the hill. Good workout for me and he loved it. Now he wears the same size shoe I do. Sigh!
Mt Pinos is above Frazier Park.
Also, Big Bear would work well and you could use the chair lift to help take the sting out of the uphills. I know some of the trails off the top may be over her limits, but even the fireroads leading to the Town Trail are pretty nice. All this is just behind Snow Summit.