One of my kids got PO'd at me when I said he could not pierce his tongue, and painted a swastika on the wall of his bedroom. (He was 14 at the time).
I grounded him to his bedroom with a toothbrush, a can of cleaner, and a copy of "Schindler's List", and I told him he could not go anywhere but back and forth to school until the wall was clean and the book was read, and a satisfactory oral report was given to me on why a kid with a Jewish grandfather should know better.
We don't take kindly to little Nazis "going through a phase" in our house. (But then I am known as the Mum who nearly knocked Jacques Villeneuve off his paddock bike for cussing at me ... )
I certainly can't see this as a harmless "phase." My dad can't even stand the color of Airedale puppies until they grow into their adult colors because it reminds him of the Nazis who came in the night to take his Dad away.
Holocaust survivors are still alive. England saved many of them and the country should be ashamed of this little twit who thought it good form to go out dressed like that. Diana had a good sense of other people's feelings and she would be ashamed of her little boy.