why not you put an other label just above you child grid and bind with you datakeys(primery key) and on row command you can call
int ParentKey = Convert.ToInt32(((Label)gvChildGrid.Parent.FindControl("lblPrimeryKey")).text)
this will give the datakeys of that perticular row of parent grid.
and for childgrid you can eaisly find on row command as commandargument.