How to add 2 child in ExpandableRecyclerView

I have created list using ExpandableRecyclerView, but I want to add 2nd child in this list at position 4. How can I add the different child in the same list?

private Activity activity;

    public RecyclerAdapterDemo(Activity activity, List<? extends ExpandableGroup> groups) {
        super(groups);
        this.activity = activity;
    }

    @Override
    public OSViewHolder onCreateGroupViewHolder(ViewGroup parent, int viewType) {
        LayoutInflater inflater = (LayoutInflater) activity.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
        View view = inflater.inflate(R.layout.group_view_holder, parent, false);

        return new OSViewHolder(view);
    }

    @Override
    public PhoneViewHolder onCreateChildViewHolder(ViewGroup parent, final int viewType) {
        LayoutInflater inflater = (LayoutInflater) activity.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
        View view = inflater.inflate(R.layout.child_view_holder, parent, false);

        return new PhoneViewHolder(view);
    }

    @Override
    public void onBindChildViewHolder(PhoneViewHolder holder, int flatPosition, ExpandableGroup group, int childIndex) {
        final Phone phone = ((MobileOS)group).getItems().get(childIndex);
        holder.onBind(phone,group);
    }

    @Override
    public void onBindGroupViewHolder(OSViewHolder holder, int flatPosition, ExpandableGroup group) {
        holder.setGroupName(group);
    }

Hope this will help you. Just make some research you will find solution.