Matting moving text over an image

I have been trying several options and belive this must be simple, but I am missing how to create a matte for animated text.

What I want to do it have text over an image and have the text appear that is is coming from behind an element in the image.

For example, there is picture of a child sitting on a log, I want the text to animate over the image, but you only see the animated text after it passes the child's arm. The end effect is the appearance that the text is coming from behind the child, but in front of the background. I hope that makes sense.

Any ideas are greatly appreciated.