Universal only has one drop off area, the 2 Universal parks are next to each other, not like Disney where you need to be dropped at a particular park.
Is it just the 3 of you or is there another adult? The 2 big rides, Escape from Gringotts and Forbidden Journey will probably be too tall for your 6 yo and possibly even your 9yo - FJ height restriction is 48" and Gringotts is 42" with an adult, 48" without an adult, if it is just the 3 of you, the child swap option might not be an option. If your girls are too short for those 2 rides, it will take a LOT of the pressure off any touring plans. The other 2 rides at Hodsmeade IoA are the Flight of the Hippogriff 36", and Dragon Challenge 54". Even if you can't ride FJ, you can still tour the castle in the queue, and that is a must to for a Potterhead. Dragon Challenge you can still walk into the first bit of the queue area and they have the car and the TriWizard cup.
If your 9yo is tall enough for FP, it has a couple of intense bits in it, giant spiders and swooping death eaters, though last time I went in Jan the death eaters didn't have their black cloaks on and weren't nearly as intense as they were with their robes on!
My husband is coming too. He is younger than me, so he was a child when HP came out, hence he's very excited. I think both of my girls meet the height challenge for FJ. I SO appreciate everyone's responses. Not surprisingly, I'm trying to make the most of our time (who isn't, right?), so any and all advice is welcomed with open arms. Can i bring food in? What about backpacks?