Short answer: no tariff on plastic kits at this time.
Here's way too much info about tariffs on imports. Plastic model kits are included in a huge list called the "Harmonized Tariff Schedule" (or HST). Kits come under the 8-digit code (HST8) of 95032000 (you will also see this listed as HST 9503.20). That code describes plastic kits as:
"Reduced-size (scale) models and similar recreational models, working or not"
The official list of products subject to the new China tariffs is available at:
A quick search of that document shows HST 9503.20 is not listed.
So as of now there is no additional tariff applied to plastic model kits imported from China.
However, I did not look up the codes for modeling tools, hobby paints, etc. so they may or may not be subject to the increased tariffs.
Fun Tariff Fact: Remember all those Japanese model ship kits that never came with electric motors? (you know, even though the instructions clearly showed they were included in the kit) While the Japanese plastic kits were exempt from tariffs, those motors were not. So the Japanese companies simply left them out of the kits destined for the United States.