I have had mixed luck at the 50's Prime Time. I think it really depends on both the server and the other people in the "room" (you sort of eat in "rooms" of 4-5 tables each)
Some times we have a blast there. Other guests and I were passing notes and trying to get each other in trouble with the server. We both ended up spending time in the corner.
Other times it amounts to "here is you food, better it eat all" and they walk away.
I just asked my 8 year old what her favorite non-character place to eat at WDW is and she replied "that really expensive place, you know in MGM" I said "the Brown Derby" and she said "yes, the food is really good there". Great, my kids like expensive stuff
Seriously, the Brown Derby deals with kids very well. As long as they don't have a meltdown kids will do good in there too. If you get a booth the server will even bring over a corded phone like in the old Brown Derby in Hollywood and your kids can get a call from Goofy.
-dave