I did a lot of that kind of thing when I worked at the reptile zoo. Ages from 4-15 big groups and small groups.

5th graders are going to be old enough to know to listen and respect your instructions (still always keep an eye on the buggers!)

Depending on the animal I'd always tell them a few things that were interesting about it. First I ask if anyone knew what kind of snake/lizard/whatever it is. Kids like it if they get to show off what they know.. and they like to interact. Then walk around to each kid and let them pet/hold the animal as appropriate. Make sure they know that snakes do not like their head/neck touched and not to pat or squeeze anywhere. Most kids haven't been up close with a snake and don't realise they don't like to be pet like a dog.

If any kids are afraid don't try to force them to pet it. Instead try to show them by example how calm and friendly snakes can be. A lot of kids will warm up to the snake if you let them.