I don't know if it is the simpliest (sorry Naomi I have allergy to words like simpliest, best:) way but one way is to use the relations themselves with a little code like:

* Form.refresh - or somewhere like page2.grid.uienable

lnSelect = SELECT()
SELECT child
DODEFAULT()
SELECT (m.lnSelect)

PS: It is a known bug of grid refreshing that is there for many versions and unlikely would be corrected ever.
Cetin