As the others have said, I think this is likely an issue related to the components of your craft kits. We come across this sometimes when companies are importing products that contain wool, glue, etc. While I am not at liberty to speak on such regulations, I can think of a few resources that could be helpful to you:
1) Craft & Hobby Association - This is a US based trade association that hosts the craft industry's largest trade show in the USA, The CHA Mega Show (January in Anaheim, CA). They are likely to have some contacts that could advise on this issue. If you haven't worked with CHA already, I recommend you speak to the Director of CHA-UK who could introduce you to his American counterparts. His name is Craig De Souza [email@example.com].
2) Customs Agents - A Customs Agent can clear up a lot of regulatory questions that can affect the importation of consumer goods. They are at the port of entry where your goods will enter, which you can find on the CBP website. A complete directory of the various ports of entry can be found on the CBP Web site. (Locate a Port Of Entry - Air, Land, or Sea). http://www.cbp.gov/xp/cgov/toolbox/contacts/ports/ Click the state, then the port, then call the general phone number and ask to speak with the customs agent in the sector that your goods fall within. (If you are unsure of or haven’t decided the port where your shipment will arrive, or you are looking at importing through multiple ports, you may contact a service port of entry near where she/the goods are going.)
3) Consumer Product Safety Commission - This is the US Government Agency that regulates the safety of consumer goods products sold throughout the USA.
CONSUMER PRODUCTS SAFETY COMMISSION
301-504-7923 / 800-638-2772
I think these 3 resources should be able to answer all of your questions regarding regulations and conformity/safety standards.