Another method is to have the mother threaten them with bodily harm if they don't do exactly as you say.
HE SAID IT !!!!
More seriously though, I don't have kids of my own, and I'm a 6.3foot tall guy with a 300+ pound. So let's just say, I scare the sh*t out of the kids away normally. And I have had some successful ( some terrible too, I gotta admit ) shoot with kids.
At this age, if they are not cooperating, you've got yourself some problem children ( they all are at that age) the only solution to this is to BUY the kids off in liking you. That sound terrible to say but that's pretty much the only alternative to a failed shoot. Ask the parents what they like pre-shoot, food work well, but if you know the kid like pokemon, and you start showing them your massive collection of pokeball, chances are they will like you. ( or find you weird, but if they laugh at you, that work too. Whatever it take to earn your pay.)
When you shoot younger kids, you have to get to their levels first so they are not scared of you, that's true up to about 8-10 years of age. Pre-shoot, you want to talk with the parents and ask them to bring something the kid like, and play with the kid a little, maybe just 10-15 minutes so he get used to you.
Past the age of 3-4, you can maybe even start showing them the camera ( not necessary let them touch it, but maybe just shoot the parents and show the image to the kids ) maybe even put the camera in live view and show the parents in the display. They will get an understanding of what the camera is for and will be less inclined to be scared of it.
That's pretty much it for the "tricks" I use in these cases, I hope they work for you too.