I agree. If you are going right at Christmas I'd recommend either skipping Universal until you can get back and have more time at a slower period or try to work in a second day somehow. If you're jsut going down during the season, like the beginning of December it shouldn't be too bad.
If you only care about HP stuff it will probably be doable but jsut be expecting very heavy crowds, especailly in those sections. Outside of that I'd say DisTXMom's advice is pretty good to follow. Do Diagon Alley first then ride the train over to Hogsmeade. I'd jsut make sure to ride the train back to Diagon Alley if you have time. Stay and watch the show on the Hogwarts castle then ride back to Diagon Alley. The train ride is different in each direction so get to experience both. Plus Diagon Alley looks great at night and probably even more magical with the Christmas lights and
decor.
Also, if you have time I'd check out some of the other attractions too. Primarily Spider-Man/Transformers. They are very fun and exciting rides and different from anything Disney has. Both are the same ride system so i you only have time for one that's ok. They are both differetn from each other but get to at least expereince the ride in one manner. I'd probably recommend Spider-Man (unless your family is big Transformers fans). This woudl allow you to hit Diagon Alley early and then ride the train over to Hogsmeade adn check it out. then if you have time to kill before th evening shows can walk through Suess Landing, which is decked out well for the holidays(Grinchmas), and over to the Marvel area where the Spider-Man ride is. Then can come back to Hogsmeade for the light show on the castle and othr stuff.
For food you're probably good with etiher Three Broomsticks or Leaky Cauldron, both are great but I would avoid the typical lunch/dinner times. If going to be there the whole day maybe do lunch at Leaky Cauldron before heading over to Hogsmeade, then do dinner at Three Broomsticks in teh evening. I'd also recommend either a late mroning/afternoon snack (if skipping lunch at LC) or evening treat before leaving from Florian Fortescue's ice cream shop. They are really good and have a variety of unique flavors.