Thank you for all the advice!
I ended up choosing the Mountain Hardwear Lamina 0 Bag. I thought if I went to the -15 it might make its usefulness too limited, and the 0 bag might be more versatile with a fleece liner, as suggested. I was also concerned about the extra weight and size on the -15 bag. Since my son is already 5'1'' at ten years old, I went with the long, which in Mountain Hardwear I found was not as long as other brands' long.
If the temperature rating on the bag is pretty accurate, I think we should be OK. I realize the only way to know is to actually go camping!