You can run this simple script while selecting the parent rows.
(It will, however, remove any text formatting from the children which receive the prefix ...)
Code:
tell application id "com.omnigroup.OmniOutlinerPro3"
tell front document
set lstSeldParents to (selected rows where its children is not children of its children)
repeat with oRow in lstSeldParents
set strTopic to (topic of oRow)
repeat with oChild in children of oRow
set strChildTopic to topic of oChild
if strChildTopic does not start with strTopic then
set the topic of oChild to strTopic & ": " & strChildTopic
end if
end repeat
end repeat
end tell
end tell