H.W.
I may be a bit off on this, but maybe, just maybe, the other parents have also had kids pull this stunt and are being polite by not taking the screaming child (who was also likely seen having fun with his parents up until that point) seriously.
I have seen kids be fine with their folks (I am a fairly observant person) and then they go to leave and all hell breaks loose. I'm going to play devil's advocate on this one because I HAVE seen kids do this and have actually heard the "you aren't my mom" stuff. I'm guessing other parents have had similar experiences. Plus, she was carrying a baby, with her husband and a 'family of six'-- see where I'm going with this? Families of five would be INSANE to kidnap some random kid.
Reading for context is important.
However, the right thing the mom could have done was 'gone all mom' on him-- "If you don't leave right now, there's no video games for the next week Mister!" I'm just guessing that kidnappers aren't going to be talking about withholding privileges....