They state that advance warning signs are in place. Are they ?
Unfortunately, Google Street View is not sufficiently up-to-date for us to see the layout of signs. However their photos show the signs at the entry to the restriction are in place. Yet again, it seems, restrictions have been put in place without any warnings at all, or advance publicity. However, this is not grounds for cancellations at London Tribunals who just deal in law. You passed the sign, so the contravention is made out, unless you can show the signage is deficient, and this is, unfortunately, subjective.
Did your son not know what the "flying motorbike" signs meant ?
Anyway, from your narrative, it seems the placement of the signs and advance warning signs is the issue. The main signs are there, but are the advance signs in place ? You have the absolute right in law to take them to London Tribunals. However, you must also take a "double-or-quits" gamble, as the discount option is not offered at adjudication.