After Stella had a 'chat' with Ben she said to him something like " you know what we must do now" and rolled up her sleeve for him to pinch her, he then rolled up his sleeve, so we assume she must have piched him.
Phil thinks that Bens behaviour is due to puberty and they have their wires crossed about it being a perfectly natural thing to do.
Ben had nasty marks up his arm where she'd pinched him hard. It's why he won't go swimming with his dad and why he went ape when his dad tried to go into his room as he was dressing.
When Ben said, "It hurts", Phil thought he was talking about puberty and said it was quite normal. So now I guess Ben thinks that adults hurting you physically is a normal part of growing up.