A group of eight children were involved in interviewing people for the job.
The interviewers decided that Nigel Williams would best represent the views of children in Northern Ireland.
Mr Williams will seek out the opinions of young people and pass them on to the Government.
"He doesn't talk down to young people. He speaks to you as a friend," said 14-year-old Neil Campbell, who was one of the interviewers.
Mr Williams said he would be looking into things which affect children and young people across Northern Ireland.
He intends to cover everything ranging from road safety to things which happen in schools.
Eleven-year-old Emma said she was pleased that a children's commissioner had been appointed.
"Lots of adults don't listen to young people. Now we have someone we can go to," she said.