You Do Need power plug adapter while traveling from Tonga to China.

Here is Why You Need An Adapter

The outlet and plug type used in China are A, C and I and in Tonga is I. This is how they look like.

Tonga

In Tonga, outlets of type I are in use.

Type I

China

In China, outlets of type A, C and I are in use.

Type A

Type C

Type I