Subtractive color is where you mix the primaries to get black, which is what CMY does. c.f. additive systems eg RGB where you add primaries to get white

I know. The fact that CMYK printing (or halftoning generally) uses dots implies a sort of hybrid system. "What appears as cerulean is actually a blend of cyan, magenta, yellow and black, as magnification under a microscope demonstrates." Well, the magnification shows overlapping dots, and the result appears as stippling in yellow, green, red, cyan, and black (and white between dots).

I don't think CMY crayons (or paints) would work, although the idea is entertaining. Maybe I'm wrong and it all hinges on obtaining a good magenta, which would then mix with yellow to give red and mix with cyan to give dark blue. This is apparently the case with printer inks, but it blows my finger-painting toddler mind, and I still think trying this out with a magenta crayon will lead to disappointment. (I'm going to look and see if I've got one, but I think they aren't usually sold. I'd probably have to paint with inks to try it.)

Edit: found some suitable-looking water-soluble crayons, did my best to mix blue and red via approximate magenta, cyan, and yellow, got lavender and peach. Too approximate.