I once played Santa Clause in the mall a few years ago.
I remember a kid of around 6 peed his pants and got it all over me.
However I also remember a kid of around 3 who had just recently been potty training which is mom told me when she apologized and he had diarrhea on me.
After those incidents we started using a cloth over our laps in that mall.
Not sure if others do the same but likely not.
Still it was not that big of a deal for me as I know they didn't mean to do it and I knew it was a possibility going into that job.
I also did pee on Santa Clauses lap as a kid when I was 3 years old and recently was being potty trained and I was in underwear.