Charles schulz early cartoon strip "lil folks" is in the public domain. He did not copyright it before publishing that cartoon strip. You can use those original characters but not the names.

Li'l Folks, and it is a copyright violation.

https://en.wikipedia.org/wiki/Li%27l_Folks

It isn't necessary to rip off copyrighted cartoon characters to get messages across.

Just thought it interesting that his earlier work was not copyrighted

I won’t venture to guess what the ultimate determination would be here because I’m not qualified to apply the 4-factor test provided in 17 U.S. Code § 107 - Limitations on exclusive rights: Fair use ( https://www.law.cornell.edu/uscode/text/17/107 ). I’m guessing few, if any, are qualified until we get a more settled caselaw in this area. Im not sure why there isn’t more caselaw in this area. I’m guessing there would be more suits if copyright owners were as concerned as you are about this issue or not as sure of the legal win or effect on Q score? The infringement on trademarked name “Linus” in the title is probably an easier argument.