public class ViewModel
{
   ObservableCollection<ItemToAdd> Items;

   void OnNewItems(ICollection<ItemToAdd> newitems)
   {
      Foreach(item in newitems)
      {
         var parentNode  = Items.where( x => x = newitem)
         parentNode?.Children.Add(item)
      }
   }
}

publicclassItemToAdd
{
   ObservableCollection <ItemToAdd> Childrens
}

<telerik:RadTreeListView x:Name="Tree"
                                 ItemsSource="{Binding Items}"
                                 CanUserFreezeColumns="False" 
                                 RowIndicatorVisibility="Collapsed" 
                                 ColumnWidth="auto" 
                                 AutoGenerateColumns="False" 
                                 IsDragDropEnabled="True"
                                 IsDropPreviewLineEnabled="False"
                                 ShowColumnHeaders="True"
                                 RowStyle="{StaticResource RowStyle}" 
                                 SelectionMode="Extended" 
                                 GridLinesVisibility="None"
                                 IsExpandedBinding="{Binding IsExpandedInTree, Mode=TwoWay}"">

...

            <telerik:RadTreeListView.ChildTableDefinitions>
                <telerik:TreeListViewTableDefinition ItemsSource="{Binding Children}" />
            </telerik:RadTreeListView.ChildTableDefinitions>
        </telerik:RadTreeListView>

> Main node
    > Sub Node level 1
         >Sub Node level 2
         >Sub Node level 2
    > Sub Node level 1
         >Sub Node level 2
         >Sub Node level 2
    > Sub Node level 1
         >Sub Node level 2
         >Sub Node level 2

using System.Collections.Generic;
using Telerik.Windows.SyntaxEditor.Core.Editor;
using Telerik.Windows.SyntaxEditor.Core.Tagging;
using Telerik.Windows.SyntaxEditor.Core.Text;

namespace QuickMeasuresPro
{
    class DAXTooltipTagger : TaggerBase<ToolTipTag>
    {
        private string searchWord;

        private static readonly string[] Keywords = new string[]
        {
            "APPROXIMATEDISTINCTCOUNT","AVERAGE","AVERAGEA","AVERAGEX"
        };

        private static readonly string[] Tooltips = new string[]
        {
            "APPROXIMATEDISTINCTCOUNT(<columnName>)\r\nReturns the approximate number of rows that contain distinct values in a column.",
            "AVERAGE(<column>)\r\nReturns the average (arithmetic mean) of all the numbers in a column.",
            "AVERAGEA(<column>)\r\nReturns the average (arithmetic mean) of the values in a column.",
            "AVERAGEX(<table>,<expression>)\r\nCalculates the average (arithmetic mean) of a set of expressions evaluated over a table.",
        };

        public DAXTooltipTagger(ITextDocumentEditor editor)
            : base(editor)
        {
        }

        public override IEnumerable<TagSpan<ToolTipTag>> GetTags(NormalizedSnapshotSpanCollection spans)
        {
            if (string.IsNullOrEmpty(this.searchWord))
            {
                yield break;
            }

            TextSnapshot snapshot = this.Document.CurrentSnapshot;
            foreach (TextSnapshotSpan snapshotSpan in spans)
            {
                string lineString = snapshotSpan.GetText();
                int i = 0;
                foreach (string keyword in Keywords)
                {
                    string tooltip = Tooltips[i];
                    i++;
                    int index = lineString.IndexOf(keyword);
                    while (index != -1)
                    {
                        TextSnapshotSpan tempSnapshotSpan = new TextSnapshotSpan(snapshot,
                            new Span(snapshotSpan.Start + index, keyword.Length));

                        yield return new TagSpan<ToolTipTag>(tempSnapshotSpan, new ToolTipTag(tooltip));

                        index = lineString.IndexOf(keyword, index + keyword.Length);
                    }
                }
            }
        }

        public void UpdateSearchWord(string newSearchWord)
        {
            this.searchWord = newSearchWord;
            this.CallOnTagsChanged(this.Document.CurrentSnapshot.Span);
        }
    }
}