Excited Dog bites....9 Month old son.?

I have a one and a half year old female Harrier Hound that loves to have me chase her around the yard. My 9 month old son likes to sit in the middle of us while she runs around him. When I'm done running around, usually she isn't. I've read everywhere that when I want her to stop, just to ignore the behavior. While I turn my back and start doing other things, she will go up to my son and "Bite" him. I know that she is playing because she is still excited, but it is not OK for her to do that. I just don't know how to address this kind of behavior or how to prevent it entirely. When I try to grab her to stop her from running around, she thinks that I'm still playing and continues to run around. by the time I finally get her, I AM PISSED...It takes everything in me to keep from hitting this dog after biting my son but I keep my cool. I just don't know how to get through to her. HELP!!!