Random question: henna

Is anyone familiar with henna?? I used to help my friend who practiced Hinduism with hers, but it’s been a while, and I need help!

My stepson and daughter are wanting some so I thought I’d give it a try. I know it’s best to leave it on for 12 hours, but they’re 7 and 4, so I doubt that’s going to happen. Does anyone know the minimum amount of time to leave it on for best results? I’m assuming the quality of the henna is good because it came from a local Indian store (sorry if that’s not the right terminology).

Any additional tips?