Problem: You've told your children you'd do something, and you didn't get a chance to do it. They remember and ask about it, probably none too nicely. Either it's something you really didn't want to do at that time, or you legitimately forgot about it, and now it's too late. Yet you've got a kid or two incensed that you 'went back on your word.'
Solution: (And this has been a lifesaver these last few weeks) Act as if it's a big deal to you, too, and that you are also disappointed. Then ask them to remind you later, or the next day so that this doesn't happen again. It's all about intent and power.
By doing saying you forgot and being mildly upset about it, you take yourself out of the bad-guy position. You didn't do it on purpose (and you'd be amazed at how forgiving toddlers and preschoolers are) and you are going to make amends. You didn't say no, you simply forgot. Your intentions, in their eyes, are good.
By asking them to remind you, you give them the power they are so desperately seeking at this age. It now becomes up to them to make it happen in the future. And this cycle can go on for a while. If they forget the next day, but remember in the evening when it's too late, you can say again: "Oh, no! We did forget! You'll definitely have to remind me tomorrow. Can you remind me?" And they'll be all about it.
Now, instead of you having all the power and blocking them from doing something fun or getting something good, you've shown them that you're on their side and given them the power to make what they want a reality. All they have to do is remind you at a reasonable time.
This is the most amazing discovery that I have made in my children's third year of life.