We can help our children and grandchildren to better accept disappointment while they are young by allowing them to experience it. I think this can be especially hard for us grandparents. Allowing children to experience disappointment doesn't mean we cause it and I'm not referring to devastation here either. Allowing children to experience small disappointments and teaching them how to accept those little disappointments, trains them to better deal with the bigger disappointments they will experience later in life. So often we want to shield children from disappointment so we try to prevent it or make them forget it by giving them something much bigger and better. Often when there is more than one child we try to make everything "fair" by making it equal, but equal isn't necessarily fair or what is best.
Dealing with disappointment is generally a matter of redirecting our focus in some way. At the center of disappointment is usually "self" so a part of overcoming disappointment is redirecting our focus from "self" to "other". When my three little granddaughters come over they often want to play Webkinz. Since they all want to be first we have a rotation setup so that each time they come over someone else is first. They spend a few minutes talking about who was first last time and working out what the new line-up is every time. As they do this, they are working through the disappointment and learning to accept it as well as learning to place a little more focus on the others.
Disappointment generally involves a loss so offsetting that with some sort of gain or joy can help to train children to look for something positive in the midst of disappointment. In the above example, the one who is last gets a little more time on Webkinz than the other two. This provides a little disappointment and a little joy to both the first and last position as well as making things a little unequal for all of them. I have noticed that the one who is last will sometimes remind herself that she gets a little more time instead of dwelling on not being first.
Life isn't fair and it isn't equal. We all face disappointments but we can learn to view life and disappointment from God's perspective and teach our children to do the same.
Looking unto Jesus, the author and finisher of our faith, who for the joy that was set before Him endured the cross... Hebrews 12:2
Now hope does not disappoint, because the love of God has been poured out in our hearts by the Holy Spirit who was given to us. Romans 5:5
Let nothing be done through selfish ambition or conceit, but in lowliness of mind let each esteem others better than himself. Philippians 2:3