The art style for the first Monkey Island SE was designed to look a bit like a children's storybook... this is actually not to do with the way the game is aimed. The series, and the first two games in particular, very cleverly play with the idea of childhood imagination mixed in with references to pop culture including Star Wars.
A worthy comparison is to the best episodes of The Simpsons, at least in terms of writing. A child could play and enjoy the Monkey Island series (and many have) but there are many many layers that appeal directly to adults.
If the series did have a main target, it is to adults who have the childlike alive within them... and children who are mature.