Sounds like a fun trip! Will you be able to check in early enough on the 3rd at Portofino to take advantage of the Early Park Admission hour at Universal Studios? It's not a must-do to be able to ride Gringotts with a short wait but it's so nice to be able to explore Diagon Alley without the crowds that materialize soon after regular park open. Also, looks like IOA has early park admission at 8 am on the 4th. So you will want to arrive by 6:45 or 7 am to the gates to be at the front of the line for Hagrid's. Maybe even 6:30...Definitely agree with the advice to do IOA in a loop so you don't have to backtrack too much. Express Pass is amazing and you should be able to get on most rides with a reasonable-ish wait even in the middle of the day. It really simplifies touring. Do you have park-to-park tickets? You will need one to ride the Hogwarts Express or park-hop, if that matters to you.
The holiday entertainment will still be going while you are there and I cannot recommend the parade highly enough! The area in front of the lockers for Revenge of the Mummy has a great view, especially of the tree lighting. Also, the Grinchmas show is supposed to be great and I'm really sad I missed it on my trip in November. You have to line up about 30 minutes ahead of time to guarantee a seat. The entrance is near the book store in Seuss Island and you can meet the Grinch after. There are also special holiday versions of the stage shows in Carkitt Market and the stage in Hogsmeade - Celestina Warbeck's holiday set is great and I'm still humming "My Baby Got Me A Hippogriff for Christmas". The Christmas lights at Hogwarts are lovely and I would recommend trying to see one of the last shows of the night to avoid the crowds. You can also watch the show from the bridge between Hogsmeade and Jurassic Park. It's slightly less immersive but you're not packed in so tight. Oh, and check out the secret song list for Rip Ride Rockit so your thrill seekers can pick an appropriate soundtrack
Dining tips....Hot Butterbeer is delicious. The green stand outside of Grimmauld Place on the London Waterfront sells baked potatoes with hearty toppings that are a substantial snack or small meal. Dining reservations are much less of a big deal at USO than WDW, so you should be able to make a reservation now for your group for dinner. Toothsome and Mythos were our two favorites from the last trip.