Sci Fi is about our favorite place in all of WDW and the one place we have always made ADR's for (had to cancel last summer though due to ds' cousin being sick) and the one full serve place we'll be eating at next week. I love the movie clips, the quiet atmosphere (everyone is riveted to the screen) and think the burgers, shakes, onion rings, fries, reubens and artichoke dip are all very tasty.
My nieces just celebrated their 9th birthday there. They had fun, they didn't get the clips as being scary and were surprised they were from real movies because they were so corny. These girls are never allowed to watch anything above a Disney movie and have been known to freak out when the dumb snowman from Rudolph the Red Nosed Reindeer is on the tv. But they weren't scared at Sci Fi.
However, knowing my son as I do, back when he was anything under 6, the clips might have frightened him a bit. Now the kid is going on 16 and loves scary movies, but when he was little, things I didn't think twice about would give him nightmares. Rainforest Cafe is another one he could never have handled as a very young child.
He was 9 the first time he ate there and loved it, but I did think "glad we didn't come here a few years ago".
Each kid is different.
Prime Time is also fun, but I have found their food hit or miss but enjoy the atmosphere and servers and like many WDW eateries, Sci Fi amongst them, go for the atmosphere as much or more than the food. I agree, ask the server not to be loud with your child, not all children like that sort of thing and they get that request probably pretty frequently.