thanks for the response. I have been there but nothing worked. I gave up on the child page idea for my site anyway. I only did it because I missed the massive 'Show in menu' tick-box when setting up pages I did not want in the menu...D'oh!
Not that is has solved my problem though. I have Pages Post plugin running so I can keep topic specific post on related pages only and not on the static homepage, which I have set in Pages Post to accept Uncategorized posts, which means that it for all intent and purposes acts like the blog index page, because I have left the post page setting in the plugin blank.
But it appears (as far as I can tell) as if Widget Logic (and all the other page specific archive widgets) conflicts with Pages Post, because I can not get it to take any pages in the tag box that have settings in Pages Post.