Thank you for contacting us.
As far as I understand, you would like to change the width of the child row, so that its blue frame matches the width of the parent row. If this is the case, please try the following approach:
sender, Telerik.WinControls.UI.RowFormattingEventArgs e)
If your requirement differs from my guess, please provide a screenshot of the layout you would like to achieve.
All the best,
the Telerik team