We have travelled as a family of 6 all over the world, even when the kids were little - they are now 8, 11, 14, & 15.
The only times we have sat near each other is with Southwest - go figure! We have always been able to sit 3 & 3 side by side, with no problems at all - we usually have A boarding passes and do not line up until they start preboarding.
Now, on all of our flights with assigned seats we have been scattered around the plane! Always 2, 2, & 2 or 2 & 4. The kids are really good travellers and it is fine for us.
Our theory is now, if we want to sit together as a family - fly SWA!