Way to go, Jo...
I did something like this for each 6th grade Biology class at my cousin's middle school... I took in a bunch of different animals from an exotic pet shop I used to work at and told them a bit about each one. At that age, there were kids that were genuinely interested, or completely indifferent, some afraid of the snakes, some not... some just wanting to disrupt and make a scene for their reputation, etc.... Overall it was a good experience but there were definately times when I had to stop the whole thing due to some of the mammals being frightened by all the noise.
I think you will really enjoy doing this, and so will the kids.