Christopher-John is the second-to-youngest Logan, and is seven years old. He's a "short, round boy" who wants to "remain on good terms with everyone" (1.10). He's sensitive to other people's feelings, but is usually happy. He's not a very well developed character, even though he gets a lot of screen time. He pretty much just tags along with his brothers and sister.
At six years old, Little Man is the youngest of the Logan clan. He's always "meticulously neat" (1.2), but somehow it doesn't seem an annoying trait like it would be if we were talking about your little brother. Here's how we're first introduced to him:
[Little Man was] attempting to keep the rusty Mississippi dust from swelling with each step and drifting back upon his shiny black shoes and the cuffs of his corduroy pants by lifting each foot high before setting it gently down again. (1.2)
OCD, much? He may be the perfect gentleman, but he doesn't hesitate to stand up for what he believes. He calls Miss Crocker out on the trashed schoolbooks—as well as the insulting language included in the front cover.