I'm still not understanding why the initial flight was scheduled for 6 pm. That still wouldn't have gotten them into Phoenix airport until around midnight ET - and probably not into their hotel room until around 1 a.m. ET. I know they're young and that probably isn't an issue for them. But still seems to me you'd want to arrive earlier than that.
Plus an earlier scheduled departure time allows room to resolve potential travel issues like this -- if they were scheduled to leave at noon, the five hour delay is much more manageable. Scheduling travel for that late in the day is always risky.