Change page design for multiple groups

I have loads of groups under a parent and having set them up with one design/layout/header, say I need to change all of them.

If you have a group structure of say...

  • Parent 1
    • Child 1
    • Child 2
    • Child 3
    • Child ...
    • Child 30
  • Parent 2

What options do I have to go into Child 1, get it looking exactly right, and then, as simply as possible, apply the change to all the other 29 child groups?

From looking at the controls, it seems that I can either edit all individually and manually change and publish each one.  Or I could set a default and still have to touch each group and revert the section (header/body/footer) I required.  I was hoping something that means not editing all the groups.