Several airlines offer family boarding. One problem with it, though, is the age cut-off, as
@HouCuseChickie mentioned -- it's often something like 6 years old (as with Southwest). My children may be 11 and 13, but I still don't want them seated by themselves next to potentially-creepy strangers where I can't even see them. Last time we flew Southwest, we had Early Bird with boarding assignments B1-4 and still were
barely able to sit with our kids, thanks to the fact that half of the Orlando-bound aircraft was occupied by people who took advantage of family boarding after the A group -- in some cases, 6 or 8 adults were boarding with a single child (yes, I know that's against Southwest policy, which provides for 2 adults with 1 child, but apparently it's not a policy that's enforced) -- and our kids were "too old" for us to qualify.
By the time we boarded, there were only two sets of 2 adjacent seats left on the plane, so we divided our party of 4 into 2 pairs so each of our kids could be with one of us. Even then, DH ended up giving up his seat and moving one row back (just behind our son, and therefore able to keep an eye on him) so that another father who boarded after us could sit across the aisle from his autistic and highly anxious child. The family who boarded dead last, with a couple of 10 or 12-ish aged kids, had to split up and take 4 middle seats scattered throughout the plane, because although the flight attendants asked, nobody would give up a seat so that any of them could sit together.
We don't want to ever be that family, so now we
only fly airlines where we can pick our seat ahead of time. That also means we can't fly out of the closest local airport (which only flies direct to Orlando on Southwest), and have to travel an extra 20-60 minutes further to one of the airports that has direct flights with assigned seating (JetBlue), but it's well worth the peace of mind. Travel is stressful enough without having to worry that my adolescent daughter is going to be trapped in a middle seat, far from my sight and power to intervene, between Handsy Bubba and Drunky McPottymouth for 3 hours.