I have never been to Legoland, but the only "gang" I can imagine there would be Spanky's bunch, since there is really nothing there for anyone over 10.
Remember that WDW is a city of its own, whereas the entire
Disneyland Resort is about 2 square miles in the middle of a suburb. It is definitely safe, even at night in the immediate area (within a mile). There really aren't any gangs, but there can be crime anywhere.
Knott's has more thrill rides and so attracts more older teens, but there were no gangs when we were there. It is still in a pretty good area not far from DL.
Universal is in Hollywood and I would not want to spend the night in that neighborhood, just a day trip from DL. There is not much for younger kids to do, but it was great when we were there.
DH took our 20-something kids to Magic Mountain last summer. No gangs, but definitely a rougher crowd. I would not take younger kids or anyone that might be intimidated by that type of behavior.
The tourist areas of San Diego are also fine. I am not aware of gangs but again, there can be crime anywhere so just be careful as you would traveling to any city. I don't really think there are any more gangs than in other cities with large populations.