Yeah, I agree with artk2002. It is definitely entertainment that is geared to kids that age. It's not a boring lecture about animals, it's actually quite interactive and each child has the opportunity to touch or hold each animal if they wish. They are pretty well known and popular in my area and even my son's daycare had them in for a special day of fun (on a day when DS was absent unfortunately). I did the same kind of party with the same zoo company for my daughter's 6th birthday party with her classmates in attendance as well and it went off without a hitch (tarantulas, scorpions, reptiles, Burmese pythons and all).
The invitations I sent out also indicated that this zoo company would be coming to the party (which could be a reason why so many kids did end up attending). That said, the kids should not have been expecting one kind of party only to be disappointed/bored when they got another. There was no bait and switch going on here, lol!
I understand--but my point is, that once it became clear that the majority of the party wasn't interested in the interactive demonstration going on, it would have been better to find something else for them to do rather than try and chide them for not being quiet during the demonstrations. To me, that right there says that it wasn't exactly this exciting thing, (at least to your guests) if everybody had to be quiet, sit still, and put on their listening ears. Nor do I think a six year old is really capable of understanding what kind of entertainment this is going to be. If we had received an invitation like this, we would have thought it would be something very similar to what the local amusement park here does, which doesn't require a lot of sitting and/or being quiet.
However, this is, in general, why I think planned entertainment for children's birthday parties is a bad idea. I also think it's better to stick with the guidelines of no more guests than the birthday boy or girl's age. It's definitely a "know your audience" kind of thing. If it's for the birthday girl or boy because they enjoy it, why not do it alone? What difference does it make if there's an audience or not?