If you need to remove all nodes, you can use:

ThisForm.TreeView1.object.Nodes.Clear

But if you want to remove only child nodes of particular node, you have two ways:
1. Remove node, which children you want to remove. This will remove all childrens. After that, add the same node at the same position.

NodeKey = ThisForm.TreeView1.object.Nodes("Parent").Key
NodeText = ThisForm.TreeView1.object.Nodes("Parent").Text
ThisForm.TreeView1.object.Nodes.Remove("Parent")
ThisForm.TreeView1.object.Nodes.Add(, , NodeKey, NodeText) &&If node was at root level

2. Remove all childrens with loop:

DO while !IsNull(thisform.TreeView1.OBJECT.Nodes("Parent").child)
thisform.olecontrol1.obJECT.Nodes.Remove(thisform.olecontrol1.obJECT.Nodes("Parent").child.Key)
enddo