Isn’t it amazing what kids find funny? I’ve been surprised more than once by what seems hilarious to a 2, 4 or 6 year old. And then I remember, their sense of humor is just developing. Kids this age have a challenging time understanding word play and innuendo, but they do know that burps, farts, and poop are some of the funniest things around.
I’m guessing you’ve especially had an opportunity to witness this phenomena if you have a little boy but some little girls love potty talk too. Suddenly “poopy butt” or “potty head” is their new favorite nickname for everyone.