How old are you? A lot of people say that dogs don't take kids under 14 seriously.

My GSD rarely plays like that with me, will occasionally push the envelope with my wife, and absolutely tries to walk all over my mother but is completely submissive to my father when they doggy-sit. If I let a 12-year old walk her she drags the kid all over the place (even with a pinch-collar on).

Dogs (especially mine) are super-aware of the pack order and a lot of dogs will constantly test to see if they can out-rank someone. My guess is that they either feel they out-rank you or are testing to see where they are in the hierarchy.