Nice job. I'd recommend, again, go there just after a rain, and when it's overcast. Also, try to position yourself at their eye level, or lower if possible, I realize they are short little buggers. Also watch the stray sticks and branches.

Also, "sit on them" until you get some interaction between the pups. As they grow, they will come out and play more often and even leave the den for short periods of time. If they approach you scare them off. They are pack animals and will learn that youre not one of them, and youre not really their friend. Teach them that humans only are okay at more than 20 feet away, or you will habituate them and they will end up DEAD.

Foxes that are not afraid of people tend not to flee from people and CARS etc.

You can teach them to stay away, while still photographing them, but if they don’t get that lesson now, they may not learn it until it's too late. And NEVER take any food or give them any reason to want to see you again! You may know all of this already, so excuse me for wasting your time. But when you started talking about the wife showing it gave me reason to think that those little guys might get into some trouble. It’s hard not to want to treat them like the little precious pups that they are.

Finally, watch the mother, and take signals from her. She will show you how to act and tell you when to leave and when you can stay.

Good luck, and have fun. . .