Most of the "stutters" in books aren't very realistic (and other media too). And you make a good point about commedic effect. What would you do with a book about someone that had something else...like a character in a wheelchair...that for some reason didn't have their wheelchair written very accurately, and was the butt of the jokes? I'd probably recommend just skipping books that have characters who stutter...unless they were written by actual people who stutter. If you want to introduce kids to stuttering, then inviting someone who stutters to the classroom or watching and talking about a video of a stuttering vlogger like this one https://youtu.be/XBk-TLM6Skw would be a lot better introduction.