Hi Helene! Your six-year-old is going to love the Sci-Fi Dine-In Theater! My kids think it's so fun to eat their meals while sitting in cars watching those campy movies! I'd recommend you go with the 4:25 p.m. time if you want to see Fantasmic that night. If you took the 7:30 p.m. dinner reservation, you'd arrive at Fantasmic just as it was starting and you likely would not be able to see the show. Fantasmic is a very popular show and is generally performed only a few times a week, which means the theater usually fills up very quickly. If you want a seat, you should arrive an hour (or more) before showtime. The last time I went to Fantasmic, it was a fairly uncrowded week in the Disney parks, and even so, all of the seats in the theater were gone thirty minutes before the performance started.
If you're worried about your daughter being able to wait that long for the show to start, consider buying her a snack or small toys to amuse herself. (They sell snacks, glow-in-the-dark necklaces and other such trinkets in the theater.) The crowd usually does the wave, which kids find entertaining, and you could stroll around with your daughter while another family member holds your seats.
Another option is booking the
Fantasmic Meal Package, which gives you a voucher for seating at the show so you don't have to arrive so early. However, you won't be able to book the package at Sci-Fi; it's available only at Mama Melrose's, the Hollywood Brown Derby, and Hollywood & Vine.
Hope you have a magical vacation! Come visit me again at the Moms Panel if you have more questions! I'm happy to help!