This is all very well but on some flights they will nto allow all of their seats to be prebooked, meaning some people on flights have no choice but to miss out,.
We flew with VA to Vegas. DD was 8y at the time, and there was no way she would have been fine on an 11 hour flight, sat without a parent. It's a very long time after all. However, when we tried to reserve our seats, incidently at no extra cost anyway, we couldn't. It said there were none available. We could for the return flight, but not the flight out.
So we called VA as we were concerned. They stated, on 2 or 3 different calls, that they only release a %age of seats, for pre booking, and not all. And, the prebooked seats are not guaranteed in any case. We were told to let the check in staff know on arrival that we needed 3 seats together and it would be sorted, and it was. No idea if anyone else had to be moved to accommodate this.
VA also told us that there was no way they would have a child, in this case an 8y, sat without a parent.
Re the no guarantee of prebooked seat - again I have personal experience of this. We flew to New York with friends - 4 adults, 3 children aged 3-7y. We had prebooked 7 seats together, in a row, across the plane. However, when we checked in we had been moved. We were still near by, but with 3 in the middle of one row, 2 on left in row behind, and then another 2 in row behind them. So not same configuration at all. They also would not allow 3 children sit together with no adult, so because they changed our config of seats, we had to split the children too. I assume we'd been moved to accommodate other people needing to sit together.