Nope its not selfish at all. I dont think you are thinking of yourself enough atm. I really hope you are doing this for you and not to keep him happy. I know you love him & its his birthday and all but Id have been happier if id read he was paying for you and a friend to have a girly meal out while he took care of mia and gave you a well earned break!
Its a nice idea because its his bday but I hope for the rest of the month he treats you and considers whether everything he does is selfish or not.
Hope you have a nice night and you both enjoy yourself. x