Web Forms :: Check If Child Node Exists On Treeview?

I have some data that I am putting on a treeview and have a checkbox on the page to decide if the user wants to order the nodes by date or type.

To do this I need to check if the code has already added a child node so when grouping by the type I don't put it in lots of times.

Lets say I have Type 1 and Type 2 and each have 3 items that would fall under these I want to achieve:

Type 1
Item 1
Item 2
Item 3
Type 2
Item 1
Item 2
Item 3

Instead of:

Type 1
Item 1
Type 1
Item 2

etc etc etc

I tried using the node.Contains function but it doesn't seem to exist (I am using c#).