Girls will finger their vaginas, and even insert objects. Many kids will reach down every chance they get. This exploration produces pleasurable feelings, as we are well aware. Most, if not all, two year olds will engage in some degree of this behavior. Although many parenting books refer to this as childhood masturbation, I believe the term is misleading and unfortunate.
Toddlers just do it because it feels good.
Babies will often tug on the genitals in much the same way they tug on the ears or toes. Toddlers, though, begin to recognize that the genitals are special. They are far more interesting and more fun than toes. For some children, playing with the genitals becomes a self-comforting behavior not unlike thumbsucking. For a few, this settles into a time-consuming habit that takes them away from other important play and development.