If she wants to ride the train, make sure you get a 2 park ticket. Cannot ride without one. Also, one or both of the parks has early entry into Potter lands for resort guests. I'd check for the weekend you are there, which park or if both parks have early entry. If only one park, I'd go to the other HP area. If you are driving, count some time to park in the massive garages, and then there are walk ways, and security to get into City Walk, then walking to the first park you want to enter. And, as true for most, get there at least 45 minutes early. They sometimes open the parks, and hold you just before the rides, so you can get something to eat, and use rest rooms, while being at the front of lines.
I hope you both have a great time. There's so much detail to see in both HP areas. And, yes, even if you do ride the ride in the castle, take the castle tour to look around without holding up a line. Lots of neat stuff to see in there. Check out the women's bathroom in Hogsmeade, and listen. A student spirit is in there. And even if you don't want to eat in the HP area, still look inside the restaurants, they are themed too.
Finally, remember, the express pass is date and park(s) specific, with no refunds. While price may change, I'd go and then see if you need it, as there are many places over both parks to purchase for the day, if you do need it.
It may be a somewhat crowded Sunday. The local kids will have been back to school, and they also may visit the parks on the week-ends. And check the park hours, as usually Islands is open later than the Studios. And the only nightly Potter show is on the castle, so make sure you catch it before you call it a night. I love the HP areas at night.
Just paln a little in advnace (stydying hte maps and getting the wait times app and arrive early!)
