First of all, to the OP, there was absolutely no excuse for the behavior of the line-cutters. Even more, I am sorry your children had to see it. As far as the line-cutters, may all their teeth fall out, except the ones with tooth-aches!
As far as the actions (or inaction) of the CM's, I do think the queue CM should have been more proactive - especially after your second complaint.
The CM's at the loading dock, from what you have said, strike me as "deer caught in headlights" kids who really had no clue what to do. They didn't know any of what had transpired while you were in the queue line, and they were probably trying to figure out why you and your kids were upset. By then, the abusive family had loaded and were on their way. You said that you insisted that they call security, and that they told you they would notify security? While these CM's are well trained to handle special needs guests, it would be nice if they were a little better prepared to handle unruly guests
Then you said when you got off the boat, they told your mom that security was on the scene, talking to some of the abusive family? Who told you this? Was it the same 20 year old CM who offered to escort you, or another CM? Was it one of the CM's who boarded you on to the boat?
The young man at the end who offered to escort you, maybe I am just missing something here, but I think I would go a little easier on him. I'm sorry it seems the help he was offering seemed not enough given the whole situation you were enduring, but he did see that something was going on. Even though he might have known none of the details about what took place while you were in the queue line, he offered to make sure you and your family safely made it out of the attraction.
On the other hand, if they told your mom that security was on the scene, this CM may have had a little heads up as to your predicament and probably thought security was already on the scene. He may have felt an escort would have been sufficient.
I do find it odd that security was supposedly outside talking to "some of the family" while the other half of the family was ganged up at the boat dock waiting on you and your family.
At any rate, not cool. Sorry the OP had to put up with that garbage.