Child pages not showing for one parent page only - and only on ie8 on vista

Ok, slightly protracted title but it says it all. On my page http://braeheadprimarystirling.org/ all of the tabs show child pages except the 'Info and Newsletters 2010 - 2011' one and there doesn't seem to be any reason I can find. I have replaced the parent page and the new one doesn't display child pages either.

However, if I use Firefox the child pages are there in a drop down. Also, when I access the site at school on an older OS the drop down is there. Is it possible the problem is either Vista or IE 8 or both or nothing to do with any of that??

Very confused as to why it's there in some browsers but not IE8 on Vista - also this is not just on my own computer, a parent (a human one not a page) is also having the same problem at home with that particular groups of pages - anyone got ANY idea?

I should add that I'm VERY new to all of this and have very little experience with websites.

Thanks

Sandra