As my wife and I said to each other 'You think they way you bring up your child has a massive effect - until you have your second child"

Having said that, saying "Peer groups are the dominant influence on a child’s personality and development, full stop." is over simple. Look at the children at school. They all share a peer group. Is there personality and development all aligned? Absolutely not. Diverse friendship groups develop with distinct interests and moral codes. How do you account for that?