I was going to post this in parenting but posters in that forum tend to be even less understanding than in AIBU!
My 8 year old has bouts of very antagonistic behaviour which is absolutely draining. I don't know what to do about it. If it's just me, her and her little brother, she is absolutely fine! However, when we spend the day with my parents or my have a family day out with exH (her Dad as we are still friends post-divorce) her behaviour becomes extremely difficult to deal with.
If I try to speak to the other adult who is with us, she will start singing loudly, purposely so that we can't have a conversation, she will cry and stamp for dessert, treats, toys in shops and cafés. She will stomp off in a huff and cross her arms, she will make silly noises, be loud when I ask for a little quiet in the car because the music is on, just really draining, hyperactive behaviour.
She is as good as gold when it's the three of us, like my little helper and so caring! What on earth is going on when we're with other people? Why is she behaving like this when someone else is present?