If I was the mother of the birthday boy, I would certainly have a problem!
I'd have a problem with not having been told about this other booking in advance, so that I could make an informed decision about whether to go ahead with the childrens' birthday party.
I'd have a problem with my son and his friends being exposed to the willies of 20 strangers.
I'd have a problem with any insinuation, however slight, that this is all normal and natural and that I shouldn't be such a prude.
The management of the venue would have a GIGANTIC problem if they didn't give me a full refund for my birthday party booking.