First, there should be plenty of easy to use plugins to let you add a facebook share.
If you had to ask about adding a <body> tag, then you probably don’t have the experience to even be modifying the header.php file anyway. And Weaver’s is fairly complex code to look at. But you would never need to add a <body> tag to any child theme for any theme. If their directions say that, they are uninformed. But header.php is responsible for the <head> section, and <body> is generated elsewhere.