Well Primetime is my favorite but since you're looking to try something new I'd suggest the Sci-Fi. The food is nothing fancy but good and the atmosphere can't be beat. Your kids will love it!
Personally we like Sci Fi. My kids think its awesome that we sit in a car while we eat and they like to watch the old movie clips. The food isnt amazing but we rather enjoy it, its far from bad. HTH!
Sci-Fi has a neat atmosphere but I like their lunch menu better than dinner.
Just stick to burgers and milkshakes if you go there. That said we have never had a bad experience with Mama Melrose's. That's where we are eating on our next trip in 11 days.
I don't think I've ever stayed at Hollywood Studios late enough to eat dinner! We always eat lunch at Sci Fi. I think the food is great and it's a unique and fun atmosphere. My husband and I ate at Mama Melrose once. The food was nothing to write home about...but we are from New Jersey and have very high standards when it comes to Italian food!
Sci Fi is my vote for best food at Hollywood Studios!!!