The structure found between the epiphysis and diaphysis in a child and is a site of bone growth is the plate.

The structure that is found between the epiphysis and diaphysis in a child and serves as a site for bone growth is the "Epiphyseal Plate" or "Growth Plate".

Here are the steps to understand this:

1. The human bone structure consists of several parts, including the epiphysis, diaphysis, and metaphysis.

2. The Epiphysis refers to the end part of a long bone, initially growing separately from the shaft.

3. The Diaphysis is the main or mid section (shaft) of a long bone. It is made of compact bone and therefore is very strong and durable.

4. The Epiphyseal Plate (or growth plate) is a thin layer of cartilage located between the epiphysis and diaphysis during the growth period of a child.

5. This plate is where new bone is created as the bone grows. Once the child reaches adulthood, the plate solidifies into a thin line, known as the "epiphyseal line", and bone growth ceases.