Here's what a preschooler understands: "I'm a big girl! I get to pick! Yay, ice cream!"
Here's what she doesn't: Picking one thing means she doesn't get the other one.
While kids this age relish the chance to make a decision for themselves, they're not used to it yet, and they're still learning about consequences, says Caroline Winkler, coauthor of The Pocket Parent, a guide to gentle toddler discipline. Plus, they're impulsive, so thinking through whether cherry or chocolate will taste better after two licks is too much to handle without an assist from you.
To help your child make a choice and stick with it:
Give her a heads-up. Explain that she can have only one, and when it'll be too late to trade: "Once he scoops it, you can't choose another flavor." Narrow the field. Too many options are overwhelming. Point out two or three of her best choices.
Deal with disappointment head-on. Be sympathetic but matter-of-fact: "I'm sorry you're not happy with the book you picked today. You can get a different one next time we go to the library."
Give her a second chance sometimes. Choices are hard, and everybody needs a do-over once in a while. If all her changed mind means is an extra cup in the dishwasher, just remind her that this is really it, and let the struggle go until next time.