If your kids are fans of the Disney Jr characters, do H&V. It is fun for the kids (they get them up dancing regularly), and we enjoyed the food. I had a lot of misgivings about going due to poor reviews, so I was pleasantly surprised.
If they don't watch those shows, then i would do Sci-Fi. We took our son to 50's when he was nearly 3, but the staff couldn't really do their schtick with us. He wouldn't have understood what was going on. He loved Sci-Fi last year (nearly 4), and I think he would have enjoyed it at a younger age too. Sitting in the cars and watching the movies was fun for him. The food is excellent at both of those restaurants too.