I find that the tone of the lounge can vary with the type of event that is on at the NEC and the demographic that it is aimed at.
Always worth checking before booking, which you obviously did as you knew it was quiet.
There is usually a sign outside about the no children after 6pm policy so it shouldn't have caught anyone by surprise. Disappointing to hear that things got unpleasant.
Decent breakfast in the lounge is dependable though.