Joker: One Operation Joker Vol. 2 (manga)

The Joker is about to realize that raising kids is no joke! After falling into the same vat of chemicals that turned the Joker into his evil self, Batman is transformed into a baby and Joker finds himself lost without his arch-nemesis. The Joker takes it upon himself to raise baby Bruce Wayne...but learns that fatherhood is a lot more difficult than he initially thought.