My json missing [ ] for a list in Restful Api

I was working on a restful api which should return a json data.

My service is like this:

@Path("tfs/projectRegStatus")
public class ProjectRegStatusService {      

    @GET
    @Path("/QA")
    @Produces(MediaType.APPLICATION_JSON)
    public ProjectRegStatusVO getProjectRegStatusQA(){
        return ProjectRegStatusModule.getInstance().getProjectRegResult("QA").produceNumber();
    }

And the entity ProjectRegStatusVO is like:

@XmlRootElement

public class ProjectRegStatusVO {
    private String name;
    private int colorValue;
    private List<ProjectRegStatusVO> children;
}

And my question is that when we only have one child, JSON Data should be like:

{
  "name": "SpecPool Risk",
  "colorValue": 48,
  "children": [
    {
      "name": "Hedge Sheet",
      "colorValue": 48
    }
  ]
}

This must have [] in child list, even if it only has one child.

But my result does not have [] in child list.

Could you guys help?