Well, ideally, the guests come up and say "my child had... issue and can't wait inside that long" and the greeter will then say "okay, we'll we can do x, y, z"
And let's say the guests pick the meet the rest of party at merge option. The greeter will then explain that the child and 1 caregiver can waitout here near greeter or wherever they want while the rest of the party goes into standby. The greeter will tell the party that is goung into standby that once they get to the merge point, tell the CM that they have people outside and then call the caregiver to come into the LL to meet them. The merge CM will pull them to the side and they will wait while the caregiver and the other person wait in the LL. They don't skip anyone. Then once they all meet up at merge, they get put into the line and wait the rest of the way post merge
That's the "ideal" scenario of this. Emergency Situations are more nuanced and will be handled case by case. Depending on where you exit in the line, how many need to leave, etc.