I started all my kids between 2 and 3 years old.
There were a few basic principles I followed though.
First, keep it fun.
Second, don't sacrifice doing it right for the sake of keeping it fun.
That is, DON'T let them get into bad habbits right from the start. Kids love archery and if you get them used to the fact that there are rules involved right from the start, you will have a lot fewer problems later and they will become better shots and safer shooters of ANY type of weapon. It's really not hard to get them to shoot properly and if they learn it initially, they will retain it all their life.
A piece of cardboard or plywood about 3 or 4 ft square can be a great training aid. Place it on the ground the proper distance from the target and stand the kid in the middle of it. Stand them at the proper angle to the target with their feet spread about shoulder width so they are comfortable. Let them shoot a few shots and fine tune to where they are comfortable and are shooting well. Then trace around their feet with a marker. Explain that they need to put their feet in the "footprints" each time they shoot. This helps a LOT each time you get back from retrieving arrows. You don't have to explain each time about turning sideways and all that stuff. Just gently remind them to step in the footprints and everything else falls into place. You can even adjust how open or closed their stance without them noticing just by changing the angle of the plywood in relation to the target. If you happen to shoot in your driveway like I do, you can substitute sidewalk chalk and draw right on the driveway.
As a bit of fun, instead of just tracing their feet, you can add toes and claws to make the tracks look like bigfoot, bear tracks or whatever. They get a kick out of it.
Third, for really young kids, always use real "kids arrows" like the 1/4" dowel kind that are lightweight and have a real field tip and feathers not the blunts or bullet shaped crimped on things and plastic vanes like on the wallmart arrows. The biggest thing that seems to discourage a young archer is not missing the target. It's hitting it and having the arrow bounce off or not stick right and fall out. They fly way better too. Adult arrows that were broken and then cut down to kid size and retipped are not bad with a center shot bow but with anything non center shot, they really don't go where the kid aims very well and are usually so heavy that they either fall short or dont' stick. Bigger kids can usually use them ok but I kinda always figure I try to set a kid up with arrows that are matched properly. You would be suprised at how young of an age they can actually get to be really good shots if all their arrows hit where they are pointed instead of having a bunch of mismatched junk where even if the kid shoots right, the arrows group like a shotgun blast. Keep their gear as well tuned as your own and they will be outshooting you a lot sooner than you would believe.
Fourth, NEVER make them shoot when they don't want to. If you keep it fun and praise them, they will pester you to shoot all the time.