who is yhe child at the end with tarou's mum when she was talking to his dad ?

It belongs to Tarou's younger brother, Jirou.

Thank you for clearing that up. I thought it was tarou’s child and that tarou cheated on Mishima. But can u plz tell me where it’s mentioned it’s Jirou’s?
And that was Jirou that tarous mom was talking to right?
Also is there any meaning in the last sentence by kirino ‘summer is ending’?