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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