Please note I am jesting about the fictional character and not the actor himself. I have better things to do than point and laugh at people who used to be fat (editor's note: That last sentence was a bald-faced lie).
Chunk from Goonies
Very simply the gold standard of chubby child actors.
Augustus Gloop from Willy Wonka and the Chocolate Factory
He doesn't even look fat by today's standards.
Larry Mondello from Leave it to Beaver
Larry, at left, was Beaver's ne'er-do-well, frumpy friend, who was always eating leftovers and quick with an excuse as to why he couldn't play sports after school. As my father is quick to remind me- Larry doesn't look all that chubby today, but back then he woulda been considered a total fatty (his comments inspired mine for Augustus Gloop).
This Kid from Billy Madison
BONUS Fat Kid Actor: Gary Coleman's Cheeks
I just wanna pinch 'em!
Beans from Even Stevens
Beans was the younger, quirky, corpulent neighbor to the main character Louis Stevens- played by a young Shia LaBeouf and as a tandem they could do it all.
Peter from The Cosby Show
Both cherubic and sweatsuit-attired, Peter was and is loved by all.
You are awesome if you are the first person to name (in the comments section) this nonfat kid on Bill Cosby's lap from the same scene pictured above. Very famous now yet wasn't a child star.
Bonus Animated Kid Actor: Ralph Wiggum.
Ya Gotta have Ralph.