My two sons, age 13 and 6 (two dissertations between them!), have entered a phase where they are obsessed with making sure that the other one doesn't get more of anything construed as valuable (and each is sure that the other is getting more). It drives me bonkers since both of them, as well as my wife and I, have so much more than we deserve--in both a moral and global sense.
In addition to yelling at them about it, which always works so-o-o-o well, I've come up with a solution that I think will work instantly. When Child A complains that Child B (we really should name them one of these days) got more of "X", then we give Child B even more of "X". This should take care of things rather quickly, don't you think?