As long as it’s pure henna, it’s safe for kids over 6 years old. The uncommon G6PD deficiency is why artists typically will not henna anyone under 6; older children and adults with the deficiency aren’t really at risk like infants and toddlers, though it’s recommended to keep henna to a minimum if you know you have G6PD deficiency.
Some essential oils used in henna paste can cause slight skin reactions, like mild dermatitis, in people sensitive to those oils. Most artists will have a version of henna paste for sensitive skin which they’ll use on children and pregnant women.
If you want to read more dangers and safety precautions of henna then please read our post on dangers of henna by clicking here.