Let your kids hold the herps, that way they get to be up front and it makes it a family affair.
I'd skip the latin names, but saying where they're from, what kind of teritory they live in and what they eat (maybe bring samples for the lizards) would be cool. I'd stay away from getting too detailed in morphs other then saying they come in different colours.
Be prepared for the steve irwin comparison. as for the poisonous/venomous date, I'd not even get into that, I don't feel most would care.
Keep each species to about 3 - 5 minutes and save the snakes for last. You might want a stuffed snake at first to calm anyone that's terrified of snakes. also bring up the fact the snakes just want to be left alone.