In the location's words:

The Bullenhuser Damm Memorial is a memorial site of the Hamburg Memorials and Learning Sites Foundation in memory of the victims of Nazi crimes. It commemorates 20 Jewish children and at least 28 adults who were murdered by SS men in the basement of the building on April 20, 1945. Before they were murdered, the children were abused for pseudo-medical experiments in the Neuengamme concentration camp. The memorial consists of an exhibition and a rose garden.

Sundays 10 am - 5 pm and by appointment for guided tours. The rose garden is always open.
The memorial is closed on 24.12, 25.12, 31.12 and 1.1.

More information can be found here.