If it's a permanent greeting location, they MIGHT allow you to use the DAS. I know the old GAC stated directly on it that it was not to be used for character greetings, and I believe the new DAS follows that as well. However, reports seem to be that if there is a FP entrance, you MAY be allowed to use the DAS.
For character greetings that "pop up" throughout the park (not in a permanent set building/location) - most of these are out in the open and you could probably have someone remain in the line while the child waits nearby with another adult. Just make sure the CM managing the line is aware others are nearby and will join the person standing in the queue.
Just our personal experience -- we don't do a lot of character greetings because the lines are challenging. We've found a couple of character meals work better for our needs.
Enjoy your vacation!