Often, helping kids think through their feelings like this can get them to recognize that their anticipation of what was to come was overblown, or that their frustrations are somewhat exaggerated.
3) Search for positives in negatives
Help children learn how to deal with disappointment in constructive ways. Get in the habit of saying, "Let's see if we can find something good in all of this" after every setback, whether large or small: A crushed Lego masterpiece (we can build it again, only better this time); a canceled play date (let's see what things we can find to do that will be fun with just the two of us); or a difficult episode in school (we certainly learned something about this, and we can put that knowledge to good use going forward).
You want them to be able to take this frustration and use it as motivation rather than something debilitating. Use these types of statements often when it comes to the little things in life, and they'll automatically revert to such thinking when it comes to the larger obstacles.
4) Set your sights on the future
When all else is said and done, help children move their mode of thinking towards the future in order to get their mind away from the disappointment in the present: "I know you're sad that we couldn't get together with Jesse to play. But you know what? There will be plenty of chances for you to get together with her in the future. Maybe you can start thinking about what you might do the next time we arrange for a playdate.
5) Allow kids to fret
Sometimes kids just need to stew about things, and nothing you can say will immediately improve their outlook. This is ok too. So long as you're laying the framework and helping to outline a path forward, it's not your responsibility to instantly move them towards a cheerful state or make everything better. Some disappointments hurt more than others, and part of coping is learning to live with painful feelings. You shouldn't ever try to rush them out of painful feelings. Sometimes children need to grieve. All you need to do is model the healthy paths out of these sad emotional states, so that they know how to climb out whenever they're ready.