I'd use them for trips to amusement parks, but not for daily use.  They've been around for awhile.  They come off easier than Sharpie.

Then again, DH would have to apply because most temp tattoos contain soy.  :p

Funny because we avoid temp tattoos due to dermatitis. 

I find it amusing that we avoid temp tattoos because my allergic children have twitchy skin.  Would prefer not to add a "Made In China" stamp to their poor skin.

As with bracelets, I think it needs to be something medical personnel or lay people will recognize and find, if one is incapacitated. 

if a child prefers the temp tattoo to a bracelet, then let them use it.  but you know, they do sell temporary tattoo printer paper in craft stores, so you could design your own with the phone number already on it, so you wouldn't have to write it with marker.  And it could look exactly as you want it--less obvious if that is your concern, or with a giant allergy warning if you choose.

http://www.mnn.com/family/protection-safety/blogs/safety-tattoos-identify-kids-with-severe-food-allergies

OH MY GOODNESS.

I just can't go there.