What children's book series had a kid with a mustache?

Answer 1

It could be a series of books written and illustrated by James Stevenson. The grandkids come over to Grandpa's house and he tells them a very tall family tale. It usually involves Grandpa and Uncle Wainey in a crazy situation. Now in the present day Uncle Wainey has a mustache. So when the kids listen to the story, their vision of the past events is depicted...so the little Wainey has a mustache! Titles include "That's Exactly the Way It Wasn't," "Will You Please Feed Our Cat?", and "Grandpa's Too Good Garden."

Answer 2

The children's book series with a kid who has a mustache is "Captain Underpants" by Dav Pilkey. The character with the mustache is Mr. Krupp, the principal of Jerome Horwitz Elementary School who transforms into the superhero Captain Underpants.