So I'm guessing my kids aren't the only ones going as Trolls this Halloween. Any suggestions for how to color their hair? They have very light blonde hair, and the 4 year old's is long (the 18 month old's is still baby-length pretty much). I'm looking for something that both goes on and washes out fairly easily (more concerned about the going in as it's also my daughter's bday so we will be dealing with limited time and short attention spans).