If your going to shoot children from a class, arrange a show and tell, for the most part they are curious about the gear. All be it a bit scared of it too.
Be a kid. Know the cartoons they watch and discuss them. To us it's a bit ahh, to them it has cliff hangers, villains, hero's.
I have a few toy critters from a novelty shop, that have a air bulb attached to them. You squeeze the bulb and they open there eyes wide, or my mouse squeaks and sticks out it's tongue, my lil bird chirps and fluffs it wings.
There maybe 3 inches tall, I have Velcro glued to there feet and sit them on top of my lens hood.
It may take an extra minute or so but, you get a nice relaxed smile from them, during the shoot.
The trick is to arrange to deal with one at a time till the group shot.
Then it's another game to get them all to relax, breath and blink at the same time so all eyes are open.
Watch the Key Stone Cops, that kind of scratch your head, why didn't this work, puts you on there level in there eyes.
I also cheat. I take one of the youngest members of the family with me all the time. Me!
Romper Room, Mr. Green Jeans, PBS, the moment you treat them like the age they are your lost.
One on one with the parents near by is a game of it self.
Get them near a group of there age range and they have already started the who likes who and why.
Don't ever lose your temper, or even look like your on the virge. They will test just how far they can go and if you let the teacher run the show you will get there game face.
You have to learn your own route, young children are scared to death of strange new adults entering there game zone, learn there game.
If you ever sit down and talk to a 4,5,6 year old at there level, it would scare you with what they have learned from TV, there is no one way.
If this seems a bit too much, you wait, they will school you fast on what they will or will not tolerate.
You can go in like a trident missile and get cookie cutter or take a few extra seconds in creating a group of happy faces.
Always have a helper or three, someone has to be the butt of the joke.
When you walk into a class room or the daycare and they all go, Ya! You know it's gonna be a good shoot.
It's really hard to explain and one of those things you will know short order, if you can or can not handle.
I have 4 sisters, from that 8 Nieces, 3 Nephews, 4 grand nephews and 2 grand nieces.
I was always the goto Uncle, for that matter the only uncle as it worked out, so I hade help in learning what there likes and dislikes are. I have a Grand Niece that is now 7, heaven forbid I forget and say she is six. She keeps me well up on current events of their world and I had better listen! 
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To cover this subject would take volumes and then it just may be things that work for me.
Everyone is different and that the deal, you have to remember there all different but, there is a common ground