Well, apart from the fidgety nature of children and animals, a lot depends upon the color of dog and light of immediate surroundings.
I think close up would work more nicely when you are in a room and distant when you are out of any closed area. Try to cover as much space as possible, don't leave too much dead space at either of the sides.
The last thing i would keep in mind is i would stay at a bit above dog's head level and give as much prominence to eyes as possible.