My daughter always asks us this question every time we put her to sleep.
We love giving her stuff to dream about. I often tell my kids made up stories and each time I ask them what they want to hear, my daughter almost always asks the same thing: The princess and the dragon and the King who saves her. Or, it could be about the princess in the forest and her talking animals. Again, it could be a princess and her flying horse. Always about the princess. Always will be my princess.
Tonight she told me she'll be dreaming about white flowers and blue skies. It makes her happy.
I think it's important to keep imagination alive in our kids. Books and stories are some of the many ways we can do this. We read to them always, we tell them stories, we sing made up songs. It's always fun and, dare I say it, it keeps me young.