The Eyes have it.

The size of the eyes is a good visual cue to the size/scale or age of a creature or character. The few creatures with relatively large eyes  are generally adapted to see in low light conditions- in humans the eyes a the same size and dont really grow as such- so in a child's face they appear large and in the adult- everything has grown around them and they appear smaller. Generally childlike or human baby proportions are like a pre programmed formula for cuteness or youth.