If you can brainstorm some good character qualities for each of your children, you can write an encouraging letter to each of them. Those words will build them up and cause them to want to continue displaying those positive character qualities.
Recently I found these letters, which were written in April 2008. The first two were written by me, the last two by my 7-year-old son Stephen, in response to my letter to him:
I'm glad that you're learning to focus more on your math. When you put your mind to it, you can do anything!
I'm also proud of you that you are reading your book and don't daydream as much as before.
I like the Bible studies we do together. I love to talk about God with you. It is fun to answer your questions about God.
I love you, Bryan, and I'm glad that you're my son!
Your enthusiasm for life is so fun. Your smile brightens the room. You serve other people with purity of heart, and I love that about you!
Thank you for cleaning up so fast today. Thank you for getting Rachel's bib so quickly when I asked you. You obey so fast now, and it's a delight to have you in our family.
I like how you do your math quickly and are diligent. I like how you write your verse without complaining. I also like that I can quote my James verses to you because you know them by heart.
I like how you love God, and I'm glad you're my son!
I love you, Mom, and thank you for the card you gave me. I thank you that you love me, too. Thank you, thank you!
Thank you for playing with me, Nathaniel. And I love it when you let me be police. I love you very very very very very much!