I believe that the best option in your case is to set different NewRowStyle for parent and child grids. Within its template juts remove the LocalizationManager from the TextBlock element and add your own custom text. Please note that you will loose the Localization Capabilities doing this. Also you may set the appropriate tooltip to this TextBlock as shown at the attached project.
Please run the example and let me know how this works for you!
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items