I have always believed that if I see an adult and juvenile fox together there is an obvious candidate for the first shot.....
Out a couple of weeks back for a sit, carrying my new Hornet which has yet to do the business. My old pal is with me and on this occasion he has a .22-250. The land is a valley which narrows from right to left, so he sits to the right, me to the left. My longest shot will be maybe 160 yards, his 210ish. Anyway, nothing happening and dusk drawing in, so I'm glassing the rabbits on the far side when my pal takes a shot. Looking over I see two foxes running away from him to the opposite side of the field. As they reach the far side one runs on and the other - as they often do - stops for a last look, so I put some holdover on, touch off a shot, and down it goes. 5 minutes later the other returns and is quickly taken care of by the bigger gun to my right. When we go to pick up, the first one which I shot was a well-grown cub, the second, which my pal shot, an adult. Both were dogs. It certainly looked as if the adult had returned to the field to look for the cub.
So in answer to my original thought, I would always shoot the adult first. But the circumstances forced a different tactic, and that turned out to be surprisingly successful.
Two final points. My pal is quite ill and missed his first shot which is why I saw two animals running - I certainly can forgive him that - I'm just glad I still have his company. I measured the shot I took with the Hornet - 240 yards - I will remember that for a long time, I would never deliberately go out to use the little rifle at that kind of range, but it did the business.