Geometry of kid characters

What makes characters who have no arms and no discernable expressions so popular among kids? Sponge Bob is a square, M and M's characters are circles and Pet is a triangle.

The McDonald's McNugget Buddies are ovals, and kids used to love the little Fry Guys who were round hairy mops on stilty legs--all they could ever do was play soccer. Kid squeal with they see Pet crawl across the screen. And the giant purple triangle, Barney, continues to be roundly loved.