It seems that you are using an older version of RadControls which does not expose this property yet. It would be best if you could upgrade to the latest version.
If this is not possible, you would need to loop through all visible TreeListDataItem objects and collect those which have a parent id corresponding to the id of the current item. Something like:
TreeListDataItem = RadTreeList1.Items(0) //instead of this line, access the item you need to find the children of
).ToString() = id
'use children collection
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