Many people assume that Sherlock wouldn't get on well with children. They believe that he would be unforgiving of their lack of knowledge, that he would physically tower over them whilst treating them like dim witted adults and would have no time for the sorts of activities they get up to on a daily basis.
Those assumptions couldn't be further from the truth. He is drawn to their unending curiosity about the things going on in their world. He also still shares their drive for new knowledge and finding out how things relate to each other.
The young ones also have a refreshing lack of preconceived notions and many have no brain to mouth filter which makes them excellent sources of honest information. They will tell you what they really think and will explain what happened without hiding embarrassing details. Usually at quite a loud volume. On a case they are invaluable as they see things that are often hidden by a grown-up's tunnel vision and certainty that they are right.
The detective often spends time chatting with homeless kids in the area, about what is going on in their lives and world. These are vulnerable beings that frequently do not have a lot of people to speak to them, to support them.
He may not want children for himself but that doesn't mean he can't relate to them. Like everyone, he was a child once too. Although this is sometimes conveniently forgotten.