I third SEAmom lol.
Teaching a very young child to ride works much better when they are learning correctly from the start, and are not fighting against tack that doesn't fit them and horses that don't suit them. Just like ANY form of teaching, you want to set them up for success!
Now my son rides western, (so sizes are different) but I started him in a 10" at 4. I bought him a 12" the following year, but he is only just now growing in to the 12" at 7 years old. He still rides in his 10" half the time right now.