I needed an escape a couple of weekends ago, after getting the news that my uncle had passed away. My friend Sonia was watching her son, Alex play football just down the road from my house, so I grabbed my camera and invited myself along. Poor Alex then had to put up with me taking photos of him during the break and after the match. Though he seemed like he didn't want his picture taken when I asked him to smile, he certainly was more than happy to show off with the ball when the match was over.
That's one of the things about kids, they really want to please adults, you just have to wait them out and work on their terms. I really enjoy taking pictures of children. They are actually my favorite subject! But I can see how they might intimidate some photographers. My best suggestion would be to just start taking pictures of kids being kids, and maybe after a few minutes ask for very simple things. Like "hold still and look at me, please" or "can I get a silly face?" Most of the time after a silly face, you get a really great, genuine smile!