Just like their mums teach them Chinese - you sing to them, play with them, help them tie their shoelaces and talk to them while you do it. You can't "teach" to that age - but the kids' brains are hard-wired to acquire the langugae naturally. So you just do exactly what you would do with a native speaker baby - expose them to a lot of "motherese", which is the technical term for the type of speech that adults use with children - you can see a video about it
here
A word of warning though - your real problem is not the kids but the parents. If they think that their kids are going to visit you for an hour a week and by the end of the year will be speaking English, they're wrong. At most they can expect them to start understanding a few words.
And if it's more full-immersion (for example if you're in an all-day English language nursery school), then they've also got to be prepared for two possibilities :
a) that the kids learn to understand but don't speak. Kids of that age are very pragmatic about language use. They won't start speaking unless there's a real communicative need - and certainly not just to please an adult.
b) that they do start to acquire English but that it causes some confusion with their first language. This doesn't happen with all kids, but it does with others. The confusion will gradually get ironed out as they get older, and anyway my answer to that is that 98% of two languages still beats 100% of one hands down, but it can sometimes frighten parents so that they give up.
Here are
some links that might help. It's a search page - don't stop at the first page as there's some good (better?) stuff later on.
Hope that helps.