I attend the Christmas Lectures too - I'm literally on my way home from the RI as I type :) They used to go a lot deeper and narrower, exploring a topic (eg the Standard Model or chirality) over a set of 5 hour long lectures. The target audience was scientifically aware teenage children, but they're now about engaging children for whom science might not have otherwise been an interesting topic.

My point about Simon was that he is engaging - I tried to allude to the fact that an expert wouldn't learn anything. I don't know how it comes across as a video but as a live performer he draws you in.