Next button for child pages

Hey Gang,
I have a series of pages with children that look like this:

Parent Page
– child page
– child page

Parent Page
– child page
– child page
– etc…

What I need to create a “Next Page” type button that will cycle through the child pages sequentially skipping the parents, once it reaches the last child page, I need that “Next Page” button to go to a specified page.

In theory I can see how this would work, but am lacking the PHP skills to effectively write the statement.

Any help would be GREATLY appreciated! Thanks!