You can do a couple of things. If you want to save money you can get some books in Chinese to help them learn the language. If you want more interactive but more costly way of teaching your kids chinese go to a training school. 

What age group?  Are they wanting to learn to really speak and understand? read and write? or just show off to their friends that they can count to ten and say a few phrases?

I'd recomend starting with something like ChinesePod to test the water.  It used to be free (still is if you use torrents) and is pretty effective. 

If your in China just let them play with other kids

Just monitor the bad habits they could pick up

I would get a tutor to do some fun classes once a week to get them motivated and top it up with some easy songs, learning together by pointing,drawing etc listening to some podcasts/videos. Keep it natural and engaging.