I can't find that this is always true. It does seem true for pre-boarding on single class airlines or for airlines that do not have elite priority boarding lanes. But for every airline I can find that offers an elite boarding lane, non elite pax with children or other pax with special needs get priority in the general boarding lanes.
My son was 6 weeks old when he did his first RTW. By the time his sister came along and they both grew out of the ankle bitter stage, we missed many a pre boarding call due to not well timer "nappy change toilet run". My kids grew up in airplanes and airports, so they got accustomed to delays and we always had something for them to do. Even so, kids like to move about and are not good standing still or moving slowly in a line. Airlines know this and normally try get them to the front of the boarding queue and into their seats with a "goodie package" as soon as they can.
I have flown FJ, on the Syd/Lax/Syd route in J quite a few time, so I do know what boarding in Fiji can be like. As a +3 million mile FFer, who at one time was not a WP and flew with 2 children, I do know what standing in a slowly moving boarding line with cranky kids can be like. Gotta say that if 20 or so paxs cut into the line ahead of me, well I would probably have been a bit upset. Good boarding line management should have seen those with children pulled out of the line and given what was needed to get them seated and settled on the aircraft as soon as possible. Lets trust your letter to FJ will help them to manage the boarding process with a little bit more attention to paxs with children and other special needs.
BTW it does not matter if there is one child or 50, they should board first.