Should 11 year olds dye their hair?

You don't need sun for henna, although heat will help speed up the dye release. But as long as she's indoors, not sitting outside in the cold, she can get the same color from an 8 hour application as she would in 5 hours in the warm sun. For the paste itself, sitting it on top of the fridge is just as good as sitting it in the sun to get the dye to be ready sooner.