Poster paint is as good as anything. They use it for kids in school so it isn't harmful...though I wouldn't want it in my eyes, or any other paint for that matter! The Works in most High Streets sell a range of paints suitable for use, and its very cheap too. Good luck.
wilkinsons and Tesco do a range of kids poster paints, Wilkinsons have a gold and silver in the range, might be worth a look. I have used the Orange, Green and Yellow ones easy to clean off although might stain clothing.