This might be the Telegraph getting it wrong.
During the ceremony (which was great, but yes the soldiers goose-stepping with the flag were an own goal) the BBC announced that the children were “dressed as-” or “represented-” each of the Chinese racial groups. Not “were members of-”. The Telegraph may just have got the wrong end of the stick.
You could still say the same thing if they had actually used children from the regions.
But this article uses a direct quote:
"Fifty-six children from 56 Chinese ethnic groups cluster around the Chinese national flag, representing the 56 ethnic groups," it said.