My eldest is six, and she loves being in the room when i play, and can get a note out of alto, tenor and bari. In fact, sshe managed to blow the bari with a lot more ease than my wife. However, she has no wish to play the sax - as she puts it, she doesn't want to play a musical instrument, she wants to play the drums.
Anyway, back to the point of the post. I strongly believe that a child shouldn't try to play the sax until they've got all theeir adult teeth, and are big enough to cover all keys with ease, and blow a decent amount of air though. What age that is depends on the child, but it would usually all coincide with going to secondary school. Not bad timing, as secondary schools can often be pursuaded to fund the purchase of a first sax.
Just my two pennorth.