I don't know of an exact term for that, but it looks influenced by a Japanese trend for drawing characters with excessively large, over-rounded heads and over-emphasised eyes, sometimes called chibi after a Japanese term. Apparently it's also sometimes called super-deformed, but I think that term is asking for an awkward misunderstanding... Basically, it's about going all out for cuteness and expression while maintaining some simplicity.
Those terms aren't very widely used, and are most often associated with Manga and Anime but also apply to old-school Japanese role-playing video games, which I think is at least part of where these guys are taking their inspiration.
I think the style pictured is inspired by these old-school Japanese RPGs, and also, a trend for western merchandise inspired by the chibi style:
Here's an example of a tutorial for drawing chibi-style heads. For your variant, start along those lines, make whatever deviations you want from the standard style (e.g. your example has much simpler features), then, when you've finished the head, don't draw any arms... :)