But if you think from her POV. She is going there for Tanu who is like sister to her. She couldn't go to her graduation. She can't ditch her again. They can go to picnic any other day. When it was between her kids school performance & graduation she rightfully chose kids.
Here it she didn't have to choose. She did take her kids along. Also she isn't aware of AD's plans. She did't go for AD. She call it ad's party like evryone else but Tanu's party.
So even thiugh it looks like Ad won for Bani her reasoning is valid. Only she should have let them know earlier than making them prepare for picnic.
Also she did cheat kids saying there will be lot of kids which didn't look true.