Your expand should be 'Childs.Mother', notice the 's'.
I'd like to also suggest that you modify your Child class to be the following for clarity and brevity:
public class Child
{
// Primary key
public int ChildId { get; set; }
// Foreign key
public int FatherId { get; set; }
public int MotherId { get; set; }
...
// Navigation property
public Father Father{ get; set; }
public Mother Mother { get; set; }
}
By removing the 's' from Father and Mother, as well as removing the ForeignKey attribute, you're letting EF match the relationships by convention.