Batman hadn't liked the idea of Nightwing and Robin coming along, but Nightwing had insisted. He was the one with the address, and Robin was his partner as much as she was Batman's. He'd put his foot down as far as the younger Robin went, though; he was a few blocks away, waiting for a call for backup that would probably never come.
Sneaking inside probably wouldn't have been the least bit difficult if the Joker hadn't had a hostage. The three of them could have taken out all the idiots suicidal enough to work for Joker without being noticed. But the point was to be noticed, so Nightwing was the first to drop in the middle of a small group and say, "Hi," before knocking them all out.
They hadn't raised an alarm. Damn. Nightwing sighed. "IT'S THE BATS!" he shouted in his best surprised guard voice. "GET 'EM!"
Robin soon joined him, but Batman remained in the shadows.