I'm moving in with my significant other who has a 12-year-old daughter -- she stays with us every other weekend. The child's mother is negative toward the father and myself. She constantly voices her dissatisfaction with us both on every level, even physical characteristics and heritage. She spends most of her time trying to make the child hate me and her father. The child loves her father and is withdrawn, although never rude to me. What is the best way to approach this situation?
Take the high road. Be careful to never say anything derogatory about the little girl's mother, while at the same time showing the child that you are a nice person who cares about her. Children are very perceptive, and even though this little girl may be hearing ugly things about you, she will see the way you truly are and judge for herself as she gets to know you better.
The same will be true for her father. The child loves him and will see that he is good to her and loves her no matter what her mother says. Saying something about the girl's mother will only make the two of you appear to stoop to her level.