They don't look like people, let alone children, but many characters have physical attributes such as large eyes, small breasts and noses, etc that are generally associated with children, to say nothing of personalities. This is despite the fact that they are often supposed to be older, and even adults in some (most?) anime often have child like features.
It's just a "trick" to get the audience to like the character, since what most people would consider "cute" is just an instinct similar to the "maternal instinct" which serves the continuation of the species by encouraging adults to look out and care for children, even if those children aren't their own. It's similar to body language, which is generally interpreted subconsciously, but which plays a great deal in deciding whether one person will like another.