Some Girl Peed on My Son's Head

The weather was gorgeous today so E. and I went to the park. 

Who just happened to be walking under the jungle gym at the exact spot and the exact moment a little girl decided she couldn’t hold it any longer?  That would be my son.

What are the odds?  Out of all the kids at the playground, E. was the one that got peed on.  He was un-phased by it all, but I was completely grossed out and at a loss for what to do.  I thought about going up to her mother and saying, “Excuse me, Ma’am.  Your daughter just peed on my son’s head.  I think she owes him an apology.”  But, she looked stressed out enough already.

Is this scenario in the mom handbook?  Did I miss it somehow?  I might have skipped over it because I thought it would NEVER happen.  But, I guess motherhood is full of surprises and this won’t be the last.