I have a 6 year old DD (almost 7) doing the same thing. And now my 5 year old DS is starting too. I have a feeling it's school which seems to set off the attitude change, since DD started with her horrid behaviours last year! Maybe they're realising we're not the be all and end all of their lives...I don't know, but it's not any fun that's for sure!
I am trying to mostly ignore things they say/do, but if it's something which is just an unacceptable behaviour (like DS is beginning to hit DD when he gets cranky with her, which he never used to do), I talk to them, tell them/ask them how it feels, ask their opinions of how it could have gone better, and expect that behaviour from then on. If they do the negative behavious again, I reiterate the expected behaviour and then they go to their room, or get privileges taken away.