Both my boys started with the cheapo bow with the suction cup arrows, at two years old. I took a piece of broom handle, and taped it to the bow for a riser...shaping a bit of a handle and shelf on it. I also upgraded the string, then put those rubber "no-gloves" things on it. This gave the youngsters a definite place to nock the arrows...and protected their little hands. I used these "no-gloves" for years in fact...until I could get them a glove that fit. For arrows, they used old broken arrows of mine, with rubber blunts on the ends for a long time. We made up shooting games to learn safety first, and to have fun. After a year or so, I upgraded them to the red fiberglass longbows...and they got to have some "sharp" arrows for shooting at the target as well. Only supervised shooting session, though, for many years.
As far as draw length, and poundage...this is something that I never really worried about,as most bows have the ability for them to "grow" into more weight. As they got older, I paid more attention to it. I never worried about teaching form much until the age of 5, as they really could'nt grasp alot of it. Just let them have fun with Dad!