We recommend to apply right before your event. After your hair, makeup, jewelry etc.
These are waterproof but you should avoid vigorous rubbing/scrubbing of hands, very hot water showers and contact with harsh chemicals as that might result in peeling of the henna tattoo. Avoid using moisturiser before using or on the tattoos.
Remove using nail polish remover. And for kids, you can use any oil.
No, these are only applicable on the back of the hands. They may come off if applied elsewhere.