Were I to take an 11 year old, I might take the tram, camp at Round Valley (not
Little Round Valley) and then summit.
Now, maybe your 11 y.o. is a really good hiker and the tram is too easy, but the Marion Mountain Trail is the steepest of the bunch. If Little Round Valley is definitely your destination, the Seven Pines Trail is a bit more gentle.
If you look at the below map link, the Seven Pines Trail is just north of the Marion Mtn. Trail.
http://www.parks.ca.gov/pages/636/files/MtSanJacintoSPmap.pdf