Kee,
I use 牙刷 to help my son to brush the teeth now(he has 16 teeth) with baby use 牙膏.You can buy the 牙膏 from BeBe Dream.
Actually, you cam start using the 牙刷 now.But you need to use the baby one, I bought one set from BeBe Dream which has two 牙刷, one for my son to learn how to brush the teeth(with a circle) so that he won't hurt his mouth.Another 牙刷 is for him to use later(which looks like the normal 牙刷).