Here are some ideas from an article entitled Boosting Children’s Confidence by Carleton Kendrick

Parents should often ask their children to recall positive events in their lives. Talking about your kids’ accomplishments can ground them in optimism as they grow and face new and difficult challenges and disappointments.
These discussions will remind your children of who they really are, and will build their self-esteem and resiliency. We all like to reminisce about times when we felt full of joy and achievement. These questions will help reacquaint your children with those moments.

Ask: “What have you done that has made you feel proud of yourself? Why?”
The Reason: Knowing what has given your children a sense of pride gives you insight into who they aspire to be. Their answers might surprise you. Given this information, you can help your kids discover activities that you know they’ll enjoy.

Ask: “What is your favorite family memory? Why?”
The Reason: Kids want to be valued members of their family. Your family can provide a safe place for your children to “just be themselves” and to seek shelter from the storm. It’s vital for you to know the kind of attention that your kids most want and need. It might be a one-on-one time with you, or an event with all family members.

Ask: “What was the best surprise you ever had? What made it so special?”
The Reason: Knowing how we can surprise and delight our children is profoundly important. As they discuss their “best” surprises, other family members can share their most cherished memories, too. This will help everyone learn how to give personalized, memorable surprises.

Conversation Tips
Be ready with some of your own positive remembrances. Your children may not be feeling good about themselves or their lives when you begin this discussion. They may need some vivid reminders of past achievements and good times. Telling them your recollections of their past good deeds and good times will assure them that you’ve been paying attention to their “good stuff.”