I found these little wooden stars in a craft shop (but even better would be to ask your child to draw their own stars, or you make them for them out of thick card).
We used Wax Resist painting techniques- when you draw use wax crayon, then paint over the top with a water based paint. The paint is repelled by the wax creating interesting effects. This effect isn't obvious on the wooden base we used, but they look pretty non the less.
We crudely attached masking tape and string to the back to hang them on the tree.