Proceed with what it shows initially, you may have to 'assign owners' to each ticket to get over the initial error message. On the next screen you will get an option to 'add friends & family' - you'll need your other sons fan number etc but this should allow you to add the missing family/friends...
You are right I don't deny, but in fairness the 'system' until now has only had to deal with selling a couple of thousand tickets at most for each match. In the case of say Fulham away or Charlton away then 3000 but even still over a few weeks.
Doesn't excuse it but it's not an every week...