Look at the children .They are playing soccer.这句话的同义句,

Look at the children .They are playing soccer.
这句话的同义句,

Look at the children （who are）playing soccer
保留括号里的,那么（who are）playing soccer.是定语从句.
去掉括号里的,就是动词的ing形式作后置定语.

Look at the children ,who are playing soccer.

Look at the children who are playing soccer.

Look,the children are playing soccer.