Rupert,
There are several places that the Browse and form do a file to session queue - this takes the record and fills up the Session Values. ( Generally, there is a session queue to file which updates the actual record if required.) The session value name will be the same as the field name. So if you are looking at a value in FLD:Purchase date, look at p_web.GSV('FLD:Purchasedate'). Check - it may be already loaded in the browse for the highlighted record. If it isn't, you can add some code to the Browse procedure "TakeEvent - 2 AfterRowClicked' and you can set a different value there to send to children.
Give it a try,
Ron