public class FirstClass {
 
   Set<Child> children;
}

public class SecondClass {
 
   Set<Child> children;
}

public class Child {

  Set<GrandChildren> children;

}


FisrtTable:
primary key (id);

FirstTableChild
  primary key(ch_id)
  foreign key fk_id

FirstGrandChildren
  primary key(gr_ch_id)
  foreign key fk_ch_id


SecondTable:
primary key (id)

SecondTableChild
  primary key(ch_id)
  foreign key fk_id

SecondGrandChildren
  primary key(gr_ch_id)
  foreign key fk_ch_id