It helps if you hum the "James Bond" theme song as you steathily snap the shots.I just start clicking away (if I ask, I'm afraid they will say no!) but I'm discrete about it.
I don't take any if an employee or customer is in the immediate area.
And if there is sufficient lighting, I disable the flash.
In many years of taking Christmas photos, I have only been challenged once. The key is just to choose your moment carefully, snap, then move on quickly.
(It's actually kind of fun!)