Well, I think your perspective is blinkering you and you’re overlooking how many were affected by the two world wars and how normal being bereaved was.
Milne was estranged from his son in adulthood, sure, but did not have an unhappy existence otherwise — and he would have been very surprised to be thought of as a children’s author, given his huge prolific output in other genres.
Tolkien had an unusually full and mostly happy life, being lucky with his guardians after the early deaths of his parents, very sucessful as an academic and novelist, with a talent for friendship and an unusually strong and happy marriage and four children.
Lewis, like Tolkien, his close friend, was a committed Christian and very successful academically and as a novelist and theologian, raised his wife’s sons after her death, and his brother, while an alcoholic, was also a distinguished military historian who outlived him by years.
Most of the complications of Blyton’s life were from her affairs!
JK Rowling seems to be a contented person, and hugely rich and successful.