As far as the faces go, Osamu Tezuka ( the creator of Astro Boy) created his characters with exaggerated features to make expressing emotions easier. He got this idea from Mickey Mouse. ([url=https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSt1rQEXj1-LUfIVnY_QjbTGSfKsgL_wn5RjFbpmUG0Wk8PjbEFrA]comparison[/url]) This style became popular and, as a result, was frequently mimicked. As children have proportionally larger heads than adults (we grow into our heads) and features (particularly the eyes) appear larger, this makes the characters appear more childlike. As far as body type, people in western culture often strive for a mature (not old) or sexy look. This is evident in the common practice of using 20+ actors to portray highschoolers on TV or in movies. In eastern culture, people (particularly women, but men too to an extent) value a more youthful or cute look (special emphasis should be put on the value of "cute"). This is why many pop-stars and actors have what we in the west would describe as a "baby face". The style merely reflects the culture of the target audience.

Because they are young and look like the prefect human.