Buy those kids toothpaste. DO NOT use adult toothpaste, because it contains mint so it could be very "spicy" for kids. It could scare them from using toothpaste for life....at least for a very long time anyway.
Also my bro (who is a dentist) told me that dont't start them on toothpaste till they know how to spit out. It is not so healthy for the kid to swallow too much fluoride.