Why did Miles's wife take the children and leave him?
That part of Miles's past is explained in chapter 7. The Tuck family is explaining to Winnie how they came to know that they were immortal. The Tuck family explained that they began noticing that they weren't getting hurt the way that they should be. Angus Tuck was bitten by a snake and no problems. Jesse fell out of a tree, landed on his head, and walked away. Miles was even shot, but the bullet left almost no mark. As time continued to pass though, the Tucks began noticing that the world around them was changing, but they were not changing.
But it was the passage of time that worried them most. They had worked the farm, settled down, made friends. 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.
Miles then gives Winnie details about his wife and kids.
"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 left him, because she was scared. She doesn't know why he isn't aging, but she knows something is definitely wrong with her husband. She blames it on Devil worship, but it really doesn't matter what the cause is. He creeps her out. It makes sense. I've been married for eleven years. If I looked the same now as I did when I was 23, my wife would be freaked out by me too. If she wasn't scared, then for sure she would be angry with me for not looking any older.
Miles's wife was not alone in her fear of the Tuck family though. Their friends and neighbors began to be suspicious and scared too. The Tuck family was forced to eventually flee.
"It was the same with our friends," said Mae. "They come to pull back from us. There was talk about witchcraft. Black magic. Well, you can't hardly blame them, but finally we had to leave the farm.