Our first day, we went right to Diagon Alley for the early park opening. My kids loved picking out their house-color lanyards in Quality Quidditch Supplies (Griffyndor and Hufflepuff).
I don't know how your kids are with souvenirs, but for mine, anything purchased IN the park is far superior to something purchased prior to the tip. So letting them pick out their lanyards in the park gave me a reprieve from the "I wants" for a while.
It seems like if I buy it before the trip, it's just gear for the trip. If I buy it during the trip, it's a present. Presents are always more fun than travel gear. YMMV.