Okay, I'll be the dissenting opinion. IF you and your folks can swing a deluxe room, I would definitely do for 1 night, even if you don't actually stay in the room, just for the express for 2 days. However.....WITH THE EXCEPTION OF DIAGON ALLEY....that late in August the locals have gone back to school, if you go on weekdays, I think your wait times would be manageable for most everything. Currently, express doesn't help with Forbidden Journey or the Ollivander's shop, and it's certainly possible that they won't open express on the new stuff right away, either, though that is just a guess. I would expect wait times on all HP stuff to be elevated somewhat, but likely in the neighborhood of 45-60 minute waits for Forbidden Journey, longer likely for the new Diagon Alley stuff. But those areas are at the backs of the parks, I don't really think that you would see significant wait times on the rest of the parks. I would recommend staying with your current plan, and if it looks like you will need express when you get there, you can always purchase it seperately or book a single night then.