Creating a child record from a button on the parent form and automatically return back to the parent form after the child has been saved.

You make make a summary of the minimum of the Record ID# of the root cause record.  Call it [Record ID# of the Root Cause]

Then change the button to this (not tested)

var text URLONE = URLRoot() & "db/" & [_DBID_ROOT_CAUSE] & "?a=API_GenAddRecordForm&_fid_17=" & URLEncode ([Record ID#]); 

var text URLTWO = URLRoot() & "db/"  & [_DBID_DAMAGE] & "?a=dr&rid=" & [Related Incident];

var text ADD =  

$URLONE

& "&NextURL=" & URLEncode($URLTWO);

var text EDIT = URLRoot() & "db/" & [_DBID_ROOT_CAUSE] & "?a=dr&rid=" & [Record ID# of the Root Cause];

IF([Record ID# of the Root Cause]>0, $EDIT, $ADD)