Currently, the DataFormatString property of the columns is applied only to the top-level rows and not to the child ones.

To work this around, create a new DataTemplate for the CellTemplate property of the column and set the StringFormat property of the Binding instance for the element that will be used to display the cell's value in view mode.

<telerik:GridViewDataColumn.CellTemplate>
    <DataTemplate>
        <TextBlock Text="{Binding MyPropertyValue, StringFormat=N2}"/>
    </DataTemplate>
</telerik:GridViewDataColumn.CellTemplate>