OK Jen, Here goes below my tips/tricks/advise for Universal.
Allegiant - yes, you can add bags until the day before. Also, they may offer to counter check any carryon wheelies for free if the plane will be crowded. My brother was on the Friday night flight this last week, and they weren't thrilled about the late arrival time either, but such is the price for a cheap ticket!
Universal - it will take at least a full day for each park, no matter what Skip is thinking. Also, getting back and forth down I-4 to Disney is a major haul, any time of day. The traffic on there is a nightmare, day, night or whenever! Count on a 1/2 hour...
Food recommendations - eat breakfast at your hotel, go to the park for a few hours, and then go eat a big lunch at one of the City Walk restaurants. Our favorit is Jimmy Buffet's Margaritaville. The kids all get parrot hats, and the music just feels like a giant party. DS always has to sit by the volcano to watch it "blow". And their volcano nachos are the best.


Try to avoid the City Walk restaurants for dinner as they get quite busy near park closing. Get food inside the park or go offsite.
Offiste food - I highly recommend Giordano's pizza. It's fantastic deep dish Chicago style pizza, and they have them near Universal on Sand Lake Road.
Rides - the most intense rides are HP Forbidden Journey, and Incredible Hulk coaster. Claire may not be tall enough for Hulk. Dueling Dragons is similar to Raptor at CP, except there are two different tracks.
The scariest parts about forbidden Journey are the spiders, the Dementors, and the darkness. It's not like a ride where you can see what is coming in advance, the not knowing was half the scare. But it was just about the best ride i've ever been on. DS said at 6 that it was scary, but way cool.
And sorry to disappont you, but unless you hide your camera in your pocket, they will NOT let you take it in the queue line. EVERYTHING goes in the lockers, even sunglasses. The ride is just way too crazy motion filled. You go in every direction from laying flat out on your stomach to up, down, and back. I wouldn't want to risk it.
Definitely get back to Hogsmeade right away when you get into IOA. We didn't wait for the wand show at Ollivanders, as the line for that was almost longer than FJ! If you still want to buy a wand, they sell them at a cart outside, and also at the main gift shops. They were $35 when we went last year, but high quality. You can also get scarves and other items from the houses too, those were also pricey, but also good quality.
Butterbeer - MMM definitely frozen!! Have one person wait in the line (again quite long, but worth it) while you snap pics of the outside of the shops. The candy shop is also worth a visit for a chocolate frog!
Bring Claire's autograph book, as some Dr. Seuss characters do signings.
That's all i can think of for now. Feel free to PM me if you have any questions, as I haven't been checking the DIS a whole lot lately, but I get PM notices on my e-mail on my phone.
Have fun!