Expandable Listview' checkboxes are changing values randomly

I am using expandable listview containing -

1. GroupView - with one string value and checkbox
2. ChildView - contains one string and checkbox .

If groupview is having children then drop down arrow would replace the checkbox. What is happening is, whenever I am selecting a checkbox in groupview and expanding the childview , all the checkboxes are getting selected.

I am aware of the fact that listview uses recycling of elements and just because of this behavior ,I need to maintain a boolean which will keep the state of checkbox in both group and childview , while populating the listview but my way of doing the same is not working.

Here mGroupCollection is my custom class arraylist which I am using for populating the list Items.

Below is my code for setting ListAdapter -

public class ExpandableListAdapter extends BaseExpandableListAdapter {
    private List<Category> mGroupCollection ;
    Context ctx ;
    private static final String TAG = "ExpandableListAdapter" ;

    public ExpandableListAdapter(Context ctx , List<Category> mGroupCollection){
        this.ctx = ctx ;
        this.mGroupCollection = mGroupCollection ;
    }

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

    @Override
    public Object getChild(int groupPosition, int childPosition) {
        return null;
    }

    @Override
    public int getChildrenCount(int groupPosition) {
        return mGroupCollection.get(groupPosition).subcat_array.size();
    }

    @Override
    public View getChildView(final int groupPosition, final int childPosition, boolean isLastChild, View convertView, ViewGroup parent) {
       ChildHolder childHolder ;
        if(convertView==null){
            convertView = LayoutInflater.from(ctx).inflate(R.layout.child_row,null);
            childHolder = new ChildHolder();
            childHolder.child_txt = (TextView)convertView.findViewById(R.id.child_text);
            childHolder.checkBox  = (CheckBox)convertView.findViewById(R.id.checkbox);
            convertView.setTag(childHolder);

        }else {
                childHolder = (ChildHolder)convertView.getTag();
        }

       /* if(mGroupCollection.get(groupPosition).subcat_array.get(childPosition).selected){
            childHolder.checkBox.setChecked(true);
        }
        else
            childHolder.checkBox.setChecked(false);*/

        childHolder.child_txt.setText(mGroupCollection.get(groupPosition).subcat_array.get(childPosition).subcategory_name);
        childHolder.checkBox.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
            @Override
            public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
                mGroupCollection.get(groupPosition).subcat_array.get(childPosition).selected = true ;
            }
        });
        return convertView;
    }

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

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

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

    @Override
    public View getGroupView(final int groupPosition, boolean isExpanded, View convertView, ViewGroup parent) {
        final GroupHolder holder ;
        if(convertView==null){
            convertView = LayoutInflater.from(ctx).inflate(R.layout.group_row,null);
            holder = new GroupHolder();
            holder.img = (ImageView) convertView.findViewById(R.id.bottom_arrow);
            holder.textView = (TextView)convertView.findViewById(R.id.grp_text);
            holder.chk_grp = (CheckBox)convertView.findViewById(R.id.checkbox_grp);
            convertView.setTag(holder);
        }else{
            holder = (GroupHolder)convertView.getTag();
        }

        holder.textView.setText(mGroupCollection.get(groupPosition).category_name);

        if(mGroupCollection.get(groupPosition).subcat_array.size()==0){
            holder.img.setVisibility(View.GONE);
            holder.chk_grp.setVisibility(View.VISIBLE);
        }
        else {
            holder.img.setVisibility(View.VISIBLE);
            holder.chk_grp.setVisibility(View.INVISIBLE);
        }

        if(mGroupCollection.get(groupPosition).selected){
            holder.chk_grp.setChecked(true);
        }
        else {
            Log.e(TAG,"inisde else ");
            holder.chk_grp.setChecked(false);
        }

         holder.chk_grp.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
            @Override
            public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
                mGroupCollection.get(groupPosition).selected =true ;
                Log.e(TAG,"groupPosition==="+groupPosition +",,,,ischecked==="+isChecked);
            }
        });

        if(isExpanded) {
            convertView.setBackgroundColor(ctx.getResources().getColor(R.color.colorPrimaryDark));
        }
        else{
                convertView.setBackgroundColor(ctx.getResources().getColor(R.color.colorPrimary));
            }
        return convertView;
    }

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

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

    public class GroupHolder{
        ImageView img ;
        TextView textView ;
        CheckBox chk_grp ;
    }

    private class ChildHolder {
        CheckBox checkBox ;
        TextView child_txt ;
    }



}

Any help is appreciated.Thanks.