I don't think 30 kids will attend but won't really know until rsvps.
All I can add to this thread is my personal experience - if you invite 30 kids, be ready to organize/entertain/feed/pay for 30 kids!!! It might not happen, but then again it might...
My daughter's birthday is in the middle of summer vacation, so last year we invited 20 kids, assuming a few would be away on vacation or otherwise busy (it was on a Sunday morning). NOPE! 19 showed up.
This summer we wised up and only invited 8 to our house for a slip'n'slide party. How many kids showed up? 12! Half the kids brought their siblings (which was fine - we were prepared for them).
Personally, I think 30 invitees for a 7 y.o.'s part is WAAAAY too many.

I understand wanting the invite all the kids - she wants her old friends to be there, wants to make new friends, etc., but if the party is too large it will be unmanageable and how much time will she have to get to know the new kids anyway??? Some ways you could pare it down would be to invite only the old friends, or maybe only the girls, or have her pick 7 friends...
Good luck, whatever you decide to do!
PS - The Big Bug exhibit was just here by us. It was cool to look at, but not really a birthday party kind of thing, IMHO. It's not very interactive, and you're not allowed to touch most (or all) of them. It's basically an outdoor sculpture exhibit. Your host location might be supplementing the exhibit with some activities, but I'd suggest you go see it before you finalize your plans. I think a large group of kids that age would look at the sculptures, say 'cool!', and then ask "what's next?"
