Danny was Bruce's Nanny
Danny used to be Bruce Wayne's nanny. He was looking for a job and found that Alfred needed somebody to look after Bruce while he did other things (Cleaning, cooking, getting groceries, etc.) He decided to take up the job, even though everyone had quit. But here's the thing, Bruce nor Alfred knows he's half-dead (Alfred would find out later, he's Alfred after all)
Later on, when Bruce grows up and adopts all his kids, non knowing that he had a nanny and was a troublemaker, he makes a comment about having one when he was younger. He was trying to convince his children that it wouldn't hurt to have one.
"Alfred can't always look after you. Dick and Jason are in their 20's and don't need a nanny and besides, Nannies aren't bad or weak, I had one myself after all." Bruce didn't know that this would lead to questions about who they were, what they were like, and where they lived(Tim and the others wanted to do a background check, even though they weren't there when he did have one.)
Alfred cuts in to say that his nanny was a miracle worker and always had Bruce asleep by the time he needed to be. "He always made breakfast and had Bruce up when he needed to go to school. He even helped around the manor."
On the other side of Gotham. Danny was visiting since he hadn't been around in a few years and wanted to see how things were going. Color him surprised when he finds out the little boy he nannied when he was younger is a grown man with children! Well, it had been a while since he had last seen Bruce and Alfred.
