I've used the Enable child boundaries selection function to Select all
grand-children boundaries, but I can't see the selected areas.
It's very likely that the map is currently auto-zoomed to another level of geographic area, and that your
grand-child area selection is being masked by a higher level set of boundaries. For example, you have chosen
to select all (grand-children) LSOAs within a Local Authority, but the zoom level means that the selected
LSOAs are masked by (children) MSOAs. Continue zooming in, in this case beyond MSOAs, to see your selection.
Also, confirm that
Boundary select level is set to reflects your desired level of selection, and that
Number of boundaries selected is not 0.