In order to achieve the desired by you functionality of RadAutoCompleteBox using the ChildrenOfType<T> you need to get the WaterMarkTextBox placed inside the AutoCompleteBox and set its ReadOnly property to True. Thus the TextBox part of the control will be read only (you wont' be able to type inside it) and at the same time you will be able to remove the selected items:
sender, RoutedEventArgs e)
var auto = sender
var waterMarkTextBox = auto.ChildrenOfType<RadWatermarkTextBox>().FirstOrDefault();
Hope this helps.
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