As others have said, that's really close together. Especially with 50s Prime Time, which is a very heavy "homestyle" meal, as lunch. What I would do is push Mama Melrose back to later in the day so it serves both as a meal and as a rest/break from the parks to recharge before heading off to the fireworks or Fantasmic. If, for example, the fireworks are at 8, a 6pm dinner would be both better spaced from lunch and well-timed to let your kids get their second wind before the evening shows.
As far as the restaurants themselves, 50s Prime Time is one of our family's favorites. The food and the antics are both so good, and it makes for a really fun, memorable dining experience. We've never tried Mama Melrose, partly because I've read mixed reviews but mostly because the kids also love Sci-Fi Dine-In so if we're going to have two sit-down meals at Studios on any given trip it is almost always the 50s/Sci-Fi combo.