The long and the short of it is that, as a manufacturer of items for children under 13, you need to make sure you conform to certain government regulations. They are designed to make sure that children are exposed no more than certain set levels of toxins, such as lead and other items in plastics. There are also regulations for choking hazards.
Anyways, to do this, you need to do a few things:
1. Register as a small batch manufacturer with the cpsc
2. Get tags for your items
3. If you use things such as buttons, clips, snaps, plastic headbands, etc, get a GCC (general certificate of conformity) from the manufacturer.
That's it in brief. It can be much more detailed...And anyone else, if I have made a mistake/omitted something, please correct me!