Charlotte's breakdown
Charlotte always had this picture in her head of how her family life should be. It did not include a screaming toddler or a rambunctious kiddo. She felt she had to have the perfect home, the perfect husband, the perfect kids and be the perfect housewife. When this wasn't going to plan, she felt overwhelmed and like a failure. Yes, she had a nanny to help, but she did try to be hands on as well.
I don't know if she knew kids cried and screamed for attention or what.
As for wearing the vintage skirt, she was dressing how she always dressed before. She didn't know that Lily would come up behind her to slap her bum...why would she? She didn't think that Lily would act like that.