I'm not sure the OP deserves quite the whip being handed out. They have admitted thinking the best of an LH code share flight and having lost out. The enquiry seems logical and without anger.
On previous occasions in here when the co-location of families has been discussed, it is oft asked why the computer cannot be told that 2+3 children on one ticket need to be allocated together, without waiting for check-in time, is a really simple and obvious question. In the past, I have moved for this reason and would move again BUT, what the OP is saying, is that there should be no need as any sensible planning of seats would put the five together before check-in opens.
Given that carriers always like to say how good, kind, decent and beautiful they are and that they love people to travel with them - it would be the easiest thing to advertise that families sit together with pre-allocation. If you have different surnames, then there only has to be fact of making all the reservations in one go and paying for them collectively. It is that simple.