It would have caught me off guard, too!
When kids reach a certain age, anything to do with the body or its functions becomes quite the topic of interest; as a parent, you are a bit more prepared for it (as another PP said, after being at the Zoo or Sea World with the kiddos and seeing what comes naturally to the animals, 'nough said!), but as adult that isn't around kids that much, of course the whole situation took you be surprise!
Just know that I think you are a super aunty for even taking the kids to
Disneyland, and an even better aunty that you were concerned about this.