A paperback I read in the early 80's. Family with three kids, youngest boy named Mitchell who has a dream of staying up all night long. For his birthday, his father takes him on a walking tour through the City all night long. The daughter (who I think is the narrator) has a friend named Simone or Siobhan. I think the family was working class and Dad is unemployed so times are tough.
The book is the Noonday Friends, by Mary Stoltz. The story is mostly about Franny, the daughter, and her friend Simone -- both families are broke and living in New York in a brownstone, and in one chapter the little brother gets to stay up all night as his birthday present. The father is an artist (which is why they're broke!).
Ring a bell?