How can I color my kids hair for halloween -- to dark brown?

I have a 2 year old and 3 year old who are going to be dora and diego for halloween. They will not wear wigs...They both have light brown/blondish hair. What kind of hair gel/temporary paint can I buy to make it dark brown? Can I make something that is safe and would wash out? Don't really have a lot of time to shop around so would like to buy online, or at a specific store... Any ideas? Thanks.