I don't know if this is possible with QueryOver, I worked at it for a while without getting anywhere. It is possible with LINQ:
var child = session.Query<Parent>()
.Where(p => p.Poid == y)
.SelectMany(p => p.WishesLists)
.SingleOrDefault(c => c.Poid == x);
I strongly prefer the LINQ syntax over QueryOver.
See also NH-3176