Sci-Fi is one of our must-do experiences on every trip. My boys are almost 7 and almost 5 and they always ask if I made reservations for Sci-Fi. They love eating in the cars and watching the hokey movies. The food has always been really good for us but we stick to the basics.....love love love the burgers, onion rings, and shakes. My DH loves that he can get a cherry coke for his n/a bev and the shake as his dessert! (He's just a really big kid! OK, me too.) Plus, the dim lighting and cool a/c make it an awesome break from the heat. We usually do lunch there on our DHS day and it really recharges us to finish it out.
We used to really like 50s but haven't been in a couple of years. The last two times we went, there was absolutely no schtick to it at all. No elbows off the table, no clean your plate, nothing. It used to be sooooo much better. The food is still good, but nothing we rush over for without the "entertainment." Plus, last time the service was dreadfully bad (no schtick AND no service) and we pretty much decided that since we all enjoy Sci-Fi, that's our place.
I'd give Sci-Fi a try, given your grandchildren's ages. I would bet they enjoy it! Hope you enjoy your trip.....we'll be at WDW at the same time, but we'll be at DHS a day behind you!