//Are all of today's children thick?//
I wouldn’t term them “tick” but many display a remarkable lack of general knowledge:
https://www.dailymail.co.uk/debate/article-10167795/TOM-UTLEY-shocked-woke-school-children-think-Guy-Fawkes-invented-fork.html
“…researchers found that almost a quarter of the country’s young (23 per cent) thought Bonfire Night was a pagan festival, while another 17 per cent believed it was a ‘traditional celebration to ward off evil spirits’. As for Guy Fawkes himself, 74 per cent said they had heard of him, but a tenth thought he was a fictional character, while seven per cent laboured under the delusion that he achieved fame as the inventor of fireworks. Another seven per cent ticked the box claiming he was the designer of the fork, while six per cent thought he was an MP.”
“None of this would be very surprising, perhaps, if the 1,500 people interviewed were under ten years old. But they were all aged between 16 and 29, with at least 11 years of formal education under their belts.”
This lack of general knowledge is not, in my observations, confined to Guy Fawkes.
//Do any of today's children care. Probably not. And why should they?//
They should because it is an important part of their nation’s history, ken. The Catholic/Protestant friction in England in those times gave rise to conventions which carried on well into the 20th Century (restrictions on the monarch marrying a Catholic, etc.) and played a large part in shaping the country. If they have an interest in their nation’s history they should have an interest in the Gunpowder Plot. Unfortunately too few young people have any interest in such things.
//Why on earth do the British feel the need to celebrate a failed act of Terrorism that happened over 400 years ago?//
See above. It isn’t the act itself that should necessarily be of interest but the circumstances and events which led up to it.