We are calling this the Magic Card. There is a legit name for it. But I'm too lazy to google it.
So for the Magic Card, you take note cards or cardstock. Write the best you can in white crayon. Pressing firmly. You can see it if you angle it in the light, like I had to do in order to get a picture of white crayon on white note cards.
Then paint over it using water color paint. Darker colors make it show up better. This must be done in your robe while your Mama has a cup of coffee.
As it dries, your message appears. A fun cheap project for kids.