Wow, that's a very cosmopolitan job! I envy you. I'd love to see China myself and educating kids must be rewarding.
On topic though, hm... It really depends on how old your kids are and how slow you want the singing, but i can post a few suggestions regardless.
"The Sign" by Ace of Base. It's a fun euro-pop song that, while they may not be able to handle particularly well, should still make spirits high. Think of it as a fun poppy warm up or wind down song. It should at least get them in the swing of things, enthusiastic even.
"Fix You" by Coldplay is a slow paced song, with fairly clear singing. It is however a very sad song and may not create a suitable mood for teaching. After all, you don't want to make them depressed!
"A Moment Lost" by Enya is another slow paced song, but also another sad song. It's very emotional, without having too many lyrics, and sung very clearly. It might be more useful than the above as female artists tend to sing with more definition. The words are also not drowned out by instruments, making it ideal.
"War" by Edwin Starr is another fun song that could lighten the mood. They don't have to be able to sing the whole thing, but the chorus is repeated often throughout, and i'm sure they would respond well to the lively tone. Most people can't resist simply singing along to the chorus with 'War! HOOAH! What is it good for?!". Since the theme is war (it was a protest song back in the day) it may not be suitable for young children. It is a lively and fun song despite its themes though.
"Mad World" by Gary Jules is another example of a very slow, very easy song to follow. But, like the others, it is also very sad and depressing. It might be inappropriate for children.
"Hurt" by Johnny Cash. Once again, the themes may be too heavy for children, but Cash sings so clearly and passionately that it's easy to follow along with what he's saying. An alternative to this is "Ring of Fire", which belies its themes with a fun jovial sombrero-like tune.
Other special mentions go out to "Wicked Game" by Chris Isaak, "Pictures of You" by The Cure and "Here With Me" by Dido, with the that last one perhaps being more appropriate of the three.
Unfortunately many slow songs which emphasise singing as opposed to the instruments tend to be very melancholic. It's hard to find a slow, clear song that doesn't deal with heavy themes like loss, heartbreak, depression, and so on.
