These are actually what is left after making about 3 dozen prayer rocks last year for a children's church class I teach. The cutest ones were taken. I decided to make a lot of them while I was making them and keep some on hand, because I like to give these out each year when we have our first lesson on prayer. The kids keep them on their pillow or night stand to remind them to say their prayers.
I give them a base coat of acrylic paint, usually painting them pink, blue, red, green, yellow and purple, as these seem to be the favorite kid colors. It may take 2 or 3 coats to cover the rock. Then I paint faces on them and the word "prayer" somewhere. I spray a clear acrylic coat over the top when they are dry. I like the glossy spray.
If you don't have access to suitable rocks, buy a bag from the dollar store.
I think you could give these to older kids for scripture reading, too, with the words "The Scriptures Rock".
The kids really like getting these, and it is worth the extra effort.