I also wanted to add that during our sojourn in Paris (2005, 2006 and 2007 to July 20), late June and the first half of July were much hotter than late July and August. On the other hand, I think 2004 was consistently hot through the whole summer.

Also, if you have any choice about the timing of your trip, keep in mind that Paris Plage (if they hold again in 2009 - they probably will) runs for about the last 10 days or so in July through the first three weeks in August. Going to Paris Plage, where they convert part of the banks of the Seine into a beach - with beachy amusements - might be fun for all of you.