The problem for me is that when I used them, I used too much water (I don't think I'm that different from your average five year old in this respect).  The product is marked 3 & up. My hands turned a greenish blue and it took several hand washings and a shower to finally get it off my hands.

Reminds me of when I thought (prior to starting the OTP) that I should go into the colored bubble business.  Not having any background in engineering (and having been not the strongest chemistry student either)--I added food coloring.  I had my two nieces with me (in their brand new white sneakers and pink and white outfits).  You can predict the outcome.  No one was amused that I had permanently stained their clothes and sneakers.