There’s a built-in way to do this:
self.hidesBottomBarWhenPushed = YES;
But you have to do this BEFORE the view is pushed. This is how you might want to use that:
ChildViewController* childVC = [[ChildViewController alloc] init];
childVC.hidesBottomBarWhenPushed = YES;
[self.navigationController pushViewController:childVC animated:YES];
[childVC release];