Sometimes my Thai language helper records words for me which it takes a while to figure out, especially when thai-language.com (my favourite reference) doesn't have the word. modelling clay is one of those. Once she had given me the Thai ดินน้ำมัน I was able to find out what Google Images made of it, which gives a good idea. Google translate just gives "clay" as the translation, but my language helper, when asked, said it isn't what you would use for making pottery [which is "clay"], but ดินน้ำมัน is apparently only used by children.
It is apparently made up of the components clay + oil.
Anyway, I suggest it is added to thai-language.com.