In a beautiful essay on The Toast, Mallory Ortberg argues that while Bart, Lisa and Millhouse are kids, Ralph Wiggum is the only child on the Simpsons, full of childishness that's endearing and true.
Ralph is not a rule-follower like Lisa, nor a rule-breaker like Bart; Ralph does not observe the rules because he is almost completely unaware of them. More than any of the other students at Springfield Elementary, Ralph is a child. Bart and Lisa and Milhouse and Nelson and Janey are kids, and therein lies the difference. Ralph sees things that aren't there ("Ralph, remember the time you said Snagglepuss was outside?" "He was going to the bathroom!"), eats paste, picks his nose, volunteers unprompted, nonsensical declarations ("My cat's breath smells like cat food") disguised as Zen koans. His character is sometimes written as dim-but-profound, sometimes borderline-psychotic, and occasionally developmentally disabled, but more than anything else, Ralph like what he is: a child who hasn't yet aged into a kid, which is one of the most embarrassing things a child can be.
Not Allowed In The Deep End: Ralph Wiggum's Finest Moments [Mallory Ortberg/The Toast]