I was at the riverside seating in August, and they had the dessert buffet there, as well as your choice of coffee, soft drinks, or hot chocolate. You get the dessert and the CMs serve you the drinks until Fantasmic starts. If you see the first show, you can stay for the fireworks (provided they have them), and get more drinks/snacks at that time. The riverside seating is $10 less for children ages 3-9.
You must reserve this on your credit card. You pick up the tickets at Guest Services, located to the left of the Disneyland park entrance - no later than 7:30pm. (I wouldn't wait for the afternoon to get these tickets - I'd get them as soon as I got to the park)
Balcony seating is set in the order you make your reservation. Riverside seating is first-come/first seating - however, they take names about an hour before the show starts, so the guests who check in first get the best seats first.