At the Tucks' house, why does Mae Tuck hit the man in the yellow suit after about he spoke about his plans? How does this scene mark a turning point for Winnie?

Mae Tuck hits the man in the yellow suit at the end of chapter 19. It's an incredibly violent hit, too. She doesn't punch him. Instead, she swings the shotgun and cracks his skull with the shotgun's stock. The man in the yellow suit instantly goes down.

Her strong arms swung the shotgun round her head, like a wheel. The man in the yellow suit jerked away, but it was too late. With a dull cracking sound, the stock of the shotgun smashed into the back of his skull. He dropped like a tree, his face surprised, his eyes wide open.

Unfortunately, at that very moment, the constable rides up. He witnessed the entire thing. Needless to say, the constable is quite upset, and he demands that Mae Tuck explain her actions. He wants to know why she hit the man in the yellow suit. Mae Tuck explains that she did it to stop the man from taking Winnie against her will.

The constable stood up. "What did you hit him for?" he wheezed resentfully.
"He was taking the child away," said Mae. Her voice was dull and exhausted. "He was taking the child against her will."

The moment marks a turning point for Winnie because up until this point, she isn't 100% sure if the Tucks truly mean her no harm. During earlier chapters, Winnie is constantly trying to figure out if the Tucks did indeed kidnap her or not. After Mae proves that she is willing to go to great lengths to protect Winnie, Winnie decides that she will trust in the Tucks completely. This is why she defends the Tucks and tells the constable that she went willingly with the Tuck family.

Winnie let go of Tuck's waist and turned around. Her trembling had stopped. "They didn't kidnap me," she said. "I came because I wanted to."