Chapter 7 contains the sequence when the Tuck family is telling Winnie about how they discovered their immortality. They tell her about the snake bites, the poison toadstools, and a bunch of other instances in which one of the Tucks should have been killed. Then Miles tells Winnie that it was their lack of aging that really made them (and other people) wonder what was wrong.
But after ten years, then twenty, they had to face the fact that there was something terribly wrong. None of them was getting any older.
"I was more'n forty by then," said Miles sadly. "I was married. I had two children. But, from the look of me, I was still twenty-two. My wife, she finally made up her mind I'd sold my soul to the Devil. She left me. She went away and she took the children with her."
Miles's wife was so scared of Miles that she left and took their children with her. Miles did not try to follow and find them. Winnie asks about that decision in chapter 17. She specifically wants to know why Miles didn't find them, explain the spring to them, and give them the chance to be immortal like him. Miles admits that he thought about, but he decided it would have been too "mixed up" and "just wouldn't have worked."
"Afterwards, I thought about going to find them. I wanted to, heaven knows. But, Winnie, how'd it have been if I had? My wife was nearly forty by then. And the children—well, what was the use? They'd have been near growed theirselves. They'd have had a pa close to the same age they was. No, it'd all have been so mixed up and peculiar, it just wouldn't have worked."
The eNotes question is an opinion question that asks you to judge whether or not you think that Miles made a wise decision or not. You could argue that he made the wrong decision and should have gone after his wife and kids. Your main argument could be that being together is more important than anything else and that his family would have been grateful to finally understand what was "wrong" with Miles. You could argue the other way too: Miles made the correct decision in letting his family go. It was hard for him to know that he could never be with them again, but keeping the secret of the spring is the most important thing.
"The fewer people know about the spring, he says, the fewer there are to tell about it."