ChildRowHeight can be set on the main table element of the control like this:

this.radGridView1.TableElement.ChildRowHeight = 750;

When the grid is in a hierarchy and a parent row is expanded, the inner child template will create a new table element and its ChildRowHeight is 205 which is the default value. It would be expected that the height of the inner table element is initially set as the height of the main table element of the control.

A possible workaround is to handle the ChildViewExpanded event of the grid this way:

private void RadGridView1_ChildViewExpanded(object sender, Telerik.WinControls.UI.ChildViewExpandedEventArgs e)
{
    GridTableElement tableElement = this.radGridView1.CurrentView as GridTableElement;
    if (tableElement != null && e.ParentRow.HierarchyLevel == 1)
    {
        tableElement.ChildRowHeight = 750;
        tableElement.InvalidateMeasure(true);
    }
}