Many Harvey kids have "stubbies" like Canadian beer bottles.
Ketcham's kids tend to have bell-bottom legs.
The basic forms that make up both Harvey and Ketcham kids are very similar. 
They have the same stock cartoon kid head construction.The 2 styles have some minor -but recognizable general differences - like the legs.
Then each individual character has some specific accessory design element and that's what makes him or her iconic and identifiable.Usually the hair is a specific design, as in Dennis', Joey's, Margaret's, Audrey's.
Another way to make a generic design identifiable is to have the clothes or some other surface detail be recognizable - Dot's dot dress, Margaret's glasses.Dot and Audrey are the exact same designs-same body, same face, same dress. Dot's hair is generic. Audrey's is specific. What's the main difference? Dot has dots on her dress.
Underneath these indentifiable details which enable us to instantly recognize the character, the forms are pretty similar.It's rare that a cartoon character truly has a specific design in its underlying forms. Cartoonists use the same handful of basic shapes over and over again and most are probably not even aware of it. Here's an exception.
