I don' know much, but I GOT this one. seriously:
Aerobie Squidgie.
Here's the deal:
It's soft, so soft you can roll it up like a burrito. You can throw it at a childs face, and they will not get hurt, probably laugh. Therefore, you and your child can play catch, which is 10000000x better to learn with.
It is NOT understable. It is neutral and teaches very good golf form for kids, believe it or not. It will hyzer out on throws under 100ft. it is small, very light, and extremely easy to grip for the smallest hands. It is VERY fun to throw, even for an adult disc golfer.
My son started throwing this at 4 years old, and he can now accurately hit me in the chest (playing the fade!) at 5 1/2 years old.... from 60 -70 feet away.
Any regular adult sized golf disc that is just dumbed down to light weights will still be WAAAY to big for a 4-6 year old's hands. trust me. The next step up from the squidgie is a 100g polecat, those are cool too. But seriously, get a squidgie. You can find them at Dick's sporting goods for like 6-7 bucks.