Now, they could go with Kids for the role of George and Harold or teens but either way.
John Goodman as Mr. Krupp AKA CAPTAIN UNDERPANTS
Goodman is a fantastic actor, many years in the business and the exterior for Captain Underpants and the voice for Mr. Krupp, he's very lovable and a favourite for me to portray Captain Underpants. (Although seeing him in just underwear might frighten the kids)
Now onto George and Harold, I'd rather them be kids because it seem more real for kids to be making little comic books about an overweight man wearing only underwear. So here...
George Beard - Jaden Smith
Jaden is a good actor in my books, and fits around the age I was thinking of.
Harold Hutchins - Cameron Boyce
Now I never saw Diary of a Wimpy kid but like Jaden, around the age so works for me.