Some background would probably help a bit in interpreting LotF. There was a book called The Coral island.

written in 1857. The book follows the same premise, kids get ship wrecked and is told from the PoV of a character you may recognise as Ralph. They have an opposite experience to the kids from LotF; food is everywhere, they mange to fight off native Polynesian cannibals, help in some missionary work converting people to Christianity and generally bring order and have a easy time of it. This book was taught in British primary schools, the American equivalent to elementary school; it instilled in kids of the Empire the notion of what it meant to be of the superior western race, of being born in the land of hope and glory, and the kind of imperial order inherent in them that they should aspire to.

Golding, in actuality was a school teacher, so contrary to your belief that he never met pre-teens, he most certainly did and had very up close and personal experiences with them. These experience of what kids did is very different to the ideal, model imperial child that the Coral Island was suppose to be inspiring. The behaviour he saw in schools was also something that was intrinsic to the British social system. Class in Britain is something that in my experience people from outside Britain have a hard time grasping in how much it influences our lives here, it's not just a simple income classification, the social class you are born into essentially will dictate your life. Even as late as 2002, Britain still had one of the worst class divides in education within the industrialised world.

This is why the oft brought-up article about some kids in Tonga getting stranded isn't actually as relevant as many think it is considering the Tongan education system at the time of the incident wasn't steeped in the centuries old rigid social hierarchy of imperial and post-war Britain and all the kids in that incident were already friends of the same in-group and social class. Why does this matter? The grammar schools that Golding was teaching at weren't filled with working class children and had shared experiences of poverty; rather they were filled with entitled, petulant middle class kids.

In short, Lord of the Flies was a primarily a rebuttal to the Coral Island which was saying the superior western stock just inherently instils grand imperial order wherever they go; Golding was refuting that claim that kids when left to their own devices would just do what entitled kids do and run amok, start some fights as they wrestle between them to actualize, and very poorly at that, the imperial order they were taught and raised to believe in and, more than just being a commentary on the Conrad-esque darkness in humanity, that kids brought up in strict social hierarchies and upbringings would, again left to their own, go and start their own class wars amongst each other.