In my class I generally use something quick that generates a response.
It's called 'Class/Yes' from the Whole Brain Teaching style (or Power Teaching). Basically, you say 'class' and the students say 'yes.' It sounds basic but it works wonders. The trick is that the students need to imitate whichever mannerism you use when saying 'class.' If you say it in a high pitch, they should say it in a high pitch. Same goes if you (for instance) say it like a robot.
I too teach children in China (from 3-13 years old typically). I use this with each demographic and they love it.
You can find more about Power teaching here.
Whole Brain Teaching (Power Teaching) Lesson 1 - YouTube
The Chinese teachers say something along the lines of '1, 2, 3' and the students say '4' and put their hands on their knees and sit up straight. This works fine too but I think it's too military-like.
Good luck!