My approach to a similar situation is based on having one complete list of
the potential drop-down values, plus a three-column table. The table's
columns contain the 'parent cell' setting, the point in the list at which the
associated drop-down values begin and the number of such values. Then I have
a defined name using the offset function and the values from columns two and
three of the table; the defined name is my validation list.
Ex
Apple 1 2
Tree 3 3

Granny Smith
Macintosh
Elm
Maple
Oak

So if Apple is selected, my FirstRow is 1, RowCount is 2, and my ValidList
is =offset(A1,FirstRow-1,0,RowCount,1)