You can safely henna a child... if you use art quality henna it's all natural and won't harm her or her hair.
I don't think that it'd be an enjoyable experience for a 7 year old though. No kid wants to sit around for hours with green goop in their hair.
Why does her hair need thickening?
I have a little experience with lightly waved hair, my cousin has it.
A great, light moisturizer for type 2 hair - Pure aloe vera gel.
She probably doesn't need oils. If you use any, use it sparingly and something light. Jojoba is good.
For a child I would want to keep it very simple and natural/organic if possible. A co wash once a week (may help with thickness) and a shampoo and deep condition once a week.
Jason products work, and they have a biotin line I believe. Aubrey organics is a great line too.