For those who missed it, there was an article in the West Australian newspaper very recently about the dangers of getting henna tattoos in Bali. It seems that henna is rarely used and more often than not they use some sort of concoction which is black rather than brownish red, which is true henna. Apparently there are properties in the black brew which very often cause serious infections and ultimately scarring. Quite often these problems don't surface until some days or even weeks after the work has been done. Sorry, but I can't remember the chemicals listed in the article.
Unfortunately some parents allow their children to have these drawings done and then have to struggle with the results for some time when back at home. It would probably be wise to avoid the temptation of being "cool". Infections and scarring are not cool.