Don't think security were really at fault here... based on what has been said so far
It seems like with a gig like this quite a common practice might be dropping the kids off and seeing them safely through security, then heading for a meal out or whatever and then waiting for them in the foyer at the time the gig closed... Easy to see how someone could simply walk into that area without it seeming very suspicious... move security to the outside of the foyer? Well then the same person could simply wait outside the doors instead... There's always a point of crossover between the public space and the crowds leaving the event so even the tightest of security can't really do anything