SHARING

Ever tried to get a child to share something when they clearly do not want to? Nope, it's not easy: "I had it first!"; "I just want to do something with it"; "She didn't want it"; "But it has to stay on the castle"; "No!". There's lots of good kid-reasons for not sharing and it's one fight that's hard for you to win. If you force Jessica to give Kelly the Bratz doll, you risk being on the receiving end of Jessica's wrath. But if you can't find a way to get Kelly that doll (that she deserves), Kelly may begin to plot villainous consequences for Jessica. Kelly may also begin to see the power Jessica has over you and begin using the same tactics; double trouble.
To alleviate a disagreement between children, knowing each child's personality is paramount. You also need a reasonable understanding of the circumstances. Without a clear picture you're better off letting the kids sort it out themselves. If you do know the situation, there's a way to make both Jessica and Kelly happy - most of the time. It's called diversion.
You can distract Jessica with a different toy. You could distract Kelly with a different toy, or you distract them both with something completely different. You may even have to force them both to clean up and play somewhere else. Ideally, you give them a common enemy or goal. You can become a "monster" who wants to chase them. You could tell the girls that it's time to play a card game, or read a favorite book. Or maybe it's the perfect time to go for a long walk.
It is not, however, the perfect time to go shopping with them ;)

SMARTER THAN YOU

This is the first thing you should know about your children: They ARE smarter than you. Mostly in ways that you may not recognize. If you often find yourself at the mercy of your child's will, they've got you. They've got you figured and they've also got the tools to sustain the control they have over you. A child doesn't spend a lot of time thinking about the various ways they can exert control, but they do attempt to get what they want by using different tactics. And whether they find a tantrum that works, a plea that's successful, or that guilt reaps great rewards, children will repeat the behaviour and augment it to fit a specific need. Don't doubt that your child's got you figured out - until you become wise in the same way. Figure your child out so that they get what they need, not what they want.
Most children don't even know why they want something. They are slaves to feelings and emotions, as we can be also. But adults have years full of triumphs and gritty mistakes to account for and lots of time to ponder and assess the results. Jimmy may want candy, a hug, a toy, to not brush his teeth, to watch one more movie, or your attention. But do you give Jimmy a toy because Sam has the same one? Or do you talk to Jimmy, explain the benefits of waiting, or choosing a different toy that might be a little more in line with what you'd prefer Jimmy to have. Know what Jimmy needs and you'll find a way to show Jimmy why he'd be happy with what he needs, not what he wants.