Heh, I'm at about the same stage but with one less.
I do tend to swoop in and do the evil Mommy act. Any argument that involves raised voices or physical stuff means I decide the outcome. Couldn't decide who gets the last muffin--easy, now it's me... or the dog.
So now they have lengthy, but very quiet, arguments.
If the volume goes up I ask "does evil mommy need to help you with that disagreement?" and they find they have something they agree on....