First, I'd like you to picture a dead body on the sidewalk. You don't want to picture a dead body on a sidewalk? Fine. Then, picture children laying on the sidewalk while another child draws around their body with sidewalk chalk. Better?
Now the child gets up. You probably pictured something like this, right?
Or maybe something like this if one or more children have legs broken at the knee.
Isn't it sweet how they were holding hands? And the sun was all happy and stuff.
Then, you look outside to find one small child, let's call her Baby, drawing around the shadow of the bigger child, let's call her Sissy. You would get something like this.
The children add a few finishing touches and run off to play. You look out the window and see this.
"Sissy" has free-handed a picture of "Baby's" butt. Then, realizing she might get into trouble if this was found out, she makes it into a smiling heart, forgetting the word "butt" scrawled above the picture.
Finding this quite funny, you look around at what else is drawn and see this drawing done by "Baby", returning the favor. And without fear of getting into trouble.
These children ought to be cherished for their creativity, as well as their humor. But they should never find out how funny you think their "butt drawings" are.