There are lots of factors in play here.

Where airline is actually OPERATING the flight? It might not be Delta.
Also, where are you in the boarding process - any status, seat type, etc? Those that board later may be subject to issues.

You will find this does happen from time to time. We travel monthly and always assume our carryon might get checked. We can manage with another small underseat bag if we have to.

I will say that coming from Orlando, you have bad news - lots of families with crap from Disney.
The good news is that most have so much stuff they do not do as much carryon as frequent flyers that avoid checking bags.