I've seen plenty of selfish behavior of 7 & under...
I've seen fights...
Etc.
And your "change the environment" message is not a complete cure-all.
That is true, often you can change the environment and the child won’t change because there is a physical or mental defect beyond the environment. But, nearly always, behavior is driven by environment. The Bible condemns the parent who doesn’t raise their child up to the Lord. Most often a child’s behavior reflects the parent’s behavior and response. It’s mostly environmental.
In the same way that bad doctrine leads to bad outcomes. You might like this book on parenting:
It’s extremely effective when used. It’s also the cheapest $18 you ever spent, if you want to have a family more heavenly than you can imagine. It works with spouses, friends and strangers, too.