Realistically speaking, are Kevin's family really responsible for him ending up in NY?

It's been discussed many times, but I'd say a resounding NO.

Yes, they definitely should've kept a better eye on Kevin at the airport. Better yet, Peter should've been holding his hand the whole time during the rushing sequence - that way they wouldn't have lost him.
But in the end, their mistake really only resulted in him trailing off and arriving at the wrong gate.

The airline staff are responsible for boarding the right passengers. The male steward had the right idea to only board Kevin once he located his family as he didn't have a pass, but did the stewardess actually do that?
I mean, she takes his word for it and leaves him after Kevin locates the man he assumed was Peter from a distance. So really, her negligence is what resulted in Kevin ending up in the Big Apple.
A simple "Excuse me sir. Is this your son?" would've made a world of difference.
Best case scenario: They realise Kevin's on the wrong flight, take down his details, locate his flight and board him before it leaves (hopefully).
Worst case scenario: He remains in custody of the airport security until his family can be located/contacted in Florida, then be sent on the next flight there (unless they come back first).

Then of course there's the Plaza Hotel staff, whose mishandling of the matter: Buying Kevin's "convincing story", checking him in, and direct confrontation over the credit card in the public lobby, resulting in a chase and them losing him, certainly didn't help either - the very reason Peter and Kate were mad at them.
Fortunately, Mr. Hector didn't fall for the ruse (rightfully so), but as manager he shouldn't have allowed the check-in process to go forward. Instead, he should've quietly notified police that they have an unaccompanied child in their care and advise Kevin to stay put until he can be spoken to (while also assuring him that he's not in any trouble) and the matter get's resolved.
Once the McCallister's reunite with their son and come to collect him, the hotel could take some credit for keeping Kevin safe and put the bill on Peter and Kate.


I must say, both movies have their share of incompetent adults who only exacerbate the situation(s).
In the first film, we have the donut-eating cop who wastes time asking stupid questions and when "action" is finally taken, it's only 3 knocking attempts before the case is dismissed.
I mean, the cop didn't even announce himself. A simple "Police. Anyone in there? Open up!" ain't rocket science. And if still no answer, get a warrant to search the property ffs.