Too many times these day you see children screaming and crying and getting their way despite the initial answer from their parent. When you tell your child "no" and then your child throws a fit and you give in to their demands and change your "no" to either a "yes" or a compromise, then you are building up a spoiled-brat. Yes, I said it. Some parents want to be "cool" or make sure that their child "loves them," but instead they end up with a troublemaker who does not respect them. Children disrespect their parents most often because parents allow their child to boss them around or call the shots.
My mother used to tell me, "you are a child, you don't have a choice." If you stick with whatever you say then more likely will your child grow to respect you and listen to you. There are cases in which you make a bad call or are "missing information" and there is reason to change your answer, but those should be exceptions and not the general rule. Know your place, and have your child know his or her place if you want them to be a respectful person who respects authority, including you, rather than a broken fragile adult.
Remember, you are the adult, the boss, and he or she is the child living under your roof, your rules. This goes both way, also keep your promises! Let your child trust you and know your words are true. If you think something may come up, let the child know in advance that plans may change. I tell my little nephew, "I plan to take you somewhere fun, but if your bad we won't go. If I get called into work we may have to plan another time," or, "I will get you a toy, but you have to be good or your not getting it." There have been times that boy has been bad in the store after picking out a toy and I stuck with my word and put it back on the shelf and I let him cry. I don't do this to be mean, but to teach a lesson BECAUSE I love him and show that my words are true. There is a verse in the Holy Bible from the words of Yeshua of Nazareth (Jesus Christ) that says,
"But let your ‘Yes’ be ‘Yes,’ and your ‘No,’ ‘No,’ for whatever is more than these is from the evil one." (Matthew 5:37)
What better applicable advice than the words of the Messiah of Israel, the firstborn of all creation? The context of the verse is about making oaths, but it can apply to all kinds of situations, especially parenting. Amen.