This forum gives some advice about times:
http://www.allinlondon.co.uk/knowledge/posts.p hp?thread=1737&start=135
If there have been no additional posts to that forum, by the time you read this, the first relevant post will be at the bottom of the page I've linked to (which is page 10 of numerous posts). However, as further people post, messages will move down the board. The posts I suggest you read are as follows:
1. From sarah lowman at 2304 on 7th June (currently at the bottom of page 10) ;
2. From sarah lowman (again) at 1645 on 8th June (currently on page 11) ;
3. From tori_hermione at 1724 on 9th June (also currently on page 11).
Personally, if I was taking kids into London, I could think of hundreds of less boring things to do than standing around (probably in the rain) for several hours, in the hope of getting a glimpse of one or two film stars. However, I'm not a film fan (I've not been to the cinema for over 10 years and I probably only watch around a dozen films a year on TV or DVD) so your views might well be different to mine.
Chris