Child Level Items are null

Hi, I am able to select any item in the List and save it Properly, but when I load the page again, what I want is User should be able to see previously selected item as selected rather than selected nothing. For that, it is happening when I select the parent level item but not happening if the selected item is Child.

I am using the below code to retrieve the selected item, as TaxanomyChildIdSelected value would be coming from database and depending upon that value I want to show the user which item has has selected Previously. I am using below code to do that. But FindItemByKeyValue function returns null when I am trying to retrieve the Child level item. Any help a code snippet, link or even suggestion would be very very helpful. Thanks in advance.

 TreeListDataItem item = rtlManageHealthTopics.FindItemByKeyValue("HealthTopicId", TaxanomyChildIdSelected.ToString());
                            if ((item != null) && !DBNull.Value.Equals(item))
                                item.Selected = true;


Below is the full code

WebAdmTopicHealthTopicService _objWebAdmTopicHealthTopicService = new WebAdmTopicHealthTopicService();
                    var _listOfWebAdmTopicHealthTopic = _objWebAdmTopicHealthTopicService.Find(" TopicId= " + _topic.TopicId.ToString());

                    if((_listOfWebAdmTopicHealthTopic!=null) && !DBNull.Value.Equals(_listOfWebAdmTopicHealthTopic) && (_listOfWebAdmTopicHealthTopic.Count>=1))
                    {
                         WebAdmHealthTopicViewService _objWebAdmHealthTopicViewService = new WebAdmHealthTopicViewService();
                         var _listOf_objWebAdmHealthTopicView = _objWebAdmHealthTopicViewService.Get(" HealthTopicId = " + _listOfWebAdmTopicHealthTopic[0].HealthTopicId.ToString() , " Name ");

                        if ((_listOf_objWebAdmHealthTopicView != null) && !DBNull.Value.Equals(_listOf_objWebAdmHealthTopicView) && (_listOf_objWebAdmHealthTopicView.Count() >= 1)
                            && (_listOf_objWebAdmHealthTopicView != null) && (_listOf_objWebAdmHealthTopicView[0].HealthTopicId > 0))
                        {
                            var _objOf_objWebAdmHealthTopicView = _listOf_objWebAdmHealthTopicView[0];
                            TaxanomyChildIdSelected = _listOfWebAdmTopicHealthTopic[0].HealthTopicId;                          
                            
                            TreeListDataItem item = rtlManageHealthTopics.FindItemByKeyValue("HealthTopicId", TaxanomyChildIdSelected.ToString());
                            if ((item != null) && !DBNull.Value.Equals(item))
                                item.Selected = true;                          
                        }                       
                    }