navigation : Parent , Child, Child of Child

I searched through the support forms[about 3 days worth] and didn’t find anyone fitting my need. I have a gallery page that is the parent, have two child pages under Gallery, and two more under the child page. Lets just keep this simple. A=gallery. B=child. C=child of child. When some one visits my site, I want them to be able to click on gallery and that only loads the child list [call this sub nav]. Below the sub nav I still have a gallery page to post text into. I was able to do that. Now, I just want to lock my sub nav to only show B[child’s of gallery]. Then when you click on say child page 2006, it loads the child of child page listing in the content area below. P.S. I am really new to PHP, so I might ask more questions with your responce.
Thanks for taking time out of your day.