Most tropicals will easily last a week without food, I have done this with my Rainbowfish and went on holiday for a week, did water change day before I left and fed at last min before leaving, back a week later, big water change and then fed, all perfectly ok.
In actual fact was more worried about the shrimps as had no idea if they'd be ok if left alone for a week, turns out, no problem at all.
BUT since you leaving on Dec 13th to Jan 3rd, 21 days / 3 weeks, a LONG time to go without feeding or water changes, would not risk this.
Very much doubt your Bettas and Mollies will survive without food for that long.
Are you sure there is no one you could ask to feed even just once or twice a week?
Auto feeders, I have not heard many good things about these to be honest, would not trust them really.