Problems with the onChildClick in an ExpandableListView

I'm doing a program and I do not know why it fails. I have a ExpandableListView object that makes Adapter and% Fragment that calls this object. To the elements of the list, it works, the problem is in the onChildClick , what I pretend is that when I press an element of the list, it is saved in TextView and shows it to use it later. The problem is that it only collects the first element of the list when pressed, if I pulse another element, the application stops working without giving me an error. What could I do to solve this?

Class ExpandibleListView :

public class ExpandableListAdapter extends BaseExpandableListAdapter {

    private Context _context;
    private List<String> _listDataHeader ;
    private HashMap<String, List<String>> _listDataChild;

    public ExpandableListAdapter(Context context, List<String> listDataHeader,
                                 HashMap<String, List<String>> listChildData) {
        this._context = context;
        this._listDataHeader = listDataHeader;
        this._listDataChild = listChildData;
    }

    @Override
    public Object getChild(int groupPosition, int childPosititon) {
        return this._listDataChild.get(this._listDataHeader.get(groupPosition))
                .get(childPosititon);
    }

    @Override
    public long getChildId(int groupPosition, int childPosition) {
        return childPosition;
    }

    @Override
    public View getChildView(
        int groupPosition, final int childPosition, boolean isLastChild,
        View convertView, ViewGroup parent
    ) {
        final String childText = (String) getChild(groupPosition, childPosition);

        if (convertView == null) {
            LayoutInflater infalInflater = (LayoutInflater) this._context
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = infalInflater.inflate(R.layout.list_item, null);
        }

        TextView txtListChild = (TextView) convertView
                .findViewById(R.id.lblListItem);

        txtListChild.setText(childText);
        return convertView;
    }

    @Override
    public int getChildrenCount(int groupPosition) {
        return this._listDataChild.get(this._listDataHeader.get(groupPosition))
                .size();
    }

    @Override
    public Object getGroup(int groupPosition) {
        return this._listDataHeader.get(groupPosition);
    }

    @Override
    public int getGroupCount() {
        return this._listDataHeader.size();
    }

    @Override
    public long getGroupId(int groupPosition) {
        return groupPosition;
    }

    @Override
    public View getGroupView(int groupPosition, boolean isExpanded,
                             View convertView, ViewGroup parent) {
        String headerTitle = (String) getGroup(groupPosition);
        if (convertView == null) {
            LayoutInflater infalInflater = (LayoutInflater) this._context
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = infalInflater.inflate(R.layout.list_group, null);
        }

        TextView lblListHeader = (TextView) convertView
                .findViewById(R.id.lblListHeader);
        lblListHeader.setTypeface(null, Typeface.BOLD);
        lblListHeader.setText(headerTitle);

        return convertView;
    }

    @Override
    public boolean hasStableIds() {
        return false;
    }

    @Override
    public boolean isChildSelectable(int groupPosition, int childPosition) {
        return true;
    }
}

The Fragment :

public class MainActivityFragment extends Fragment {

    //Expandable list Adapter
    ExpandableListAdapter listAdapter;
    ExpandableListView expListView;
    List<String> listDataHeader;
    HashMap<String, List<String>> listDataChild;

    //Users
    private List <User> items = new ArrayList<>();

    //Obejeto a pasar
    User user = new User();
    String password;

    @Override
    public View onCreateView(LayoutInflater inflater, final ViewGroup container,
                             Bundle savedInstanceState) {
        view = inflater.inflate(R.layout.fragment_main, container, false);

        //Botones
        TextPass = (EditText) view.findViewById(R.id.IdpasswordUser);
        TextName = (TextView) view.findViewById(R.id.text1);

        // get the listview
        expListView = (ExpandableListView) view.findViewById(R.id.lvExp);

        // preparing list data
        prepareListData();

        listAdapter = new ExpandableListAdapter(
            getActivity(), listDataHeader, listDataChild
        );

        // setting list adapter
        expListView.setAdapter(listAdapter);

        ExpandableListView ExList =(ExpandableListView) view.findViewById(R.id.lvExp) ;

        //Llamamos al boton fichar
        fichar = (Button) view.findViewById(R.id.fichar);

        //Call ExpandibleList view child on pressed
        ExList.setOnChildClickListener(new ExpandableListView.OnChildClickListener() {

            @Override
            public boolean onChildClick(
                ExpandableListView parent, View v, int groupPosition,
                int childPosition, long id
            ) {
                ShowItem(listDataHeader.get(childPosition));

                TextName.setText(
                    listAdapter
                        .getChild(groupPosition,childPosition).toString()
                );
                return false;
            }
        });
        fichar.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                password = TextPass.getText().toString();
                //SuperUsuario
                if(
                    key.equalsIgnoreCase("cesi.tic")&&
                    password.equalsIgnoreCase("123456789")
                ){
                    ((MainActivity) getActivity()).EditarUsuario();
                    fichar.setVisibility(view.INVISIBLE);
                    TextPass.setVisibility(view.INVISIBLE);
                    TextDNI.setVisibility(view.INVISIBLE);
                }else{
                    LeerFirebase();
                }
            }
        });
        return view;
    }
    public void ShowItem(String id)
    {
        // do something
    }
}

As you can see in onChildClick , it's like I think it should work but it's not like that.