its a physical change. or more accurately, a Physio-Chemical This is so because the mass of the baby is due increase in the no. of individual mass particles (like cells).

If you want the exact co-relation of the physical-chemical reactions, then read on:

The Chemical change within the body of the baby (or any living being) would be among the various bio-chemical processes that are going on. Eg. Osmoregulation, digestion, respiration (I imply the Fe-Hb relationship here). A baby eating food would be a physical ativity, but the breakdown of the food to release glucose and conversion of the glucose to glycogen would be a chemical process. However, due to the chemical processes, a very predominant physical process takes place: Division and multiplication of cells. Each cell has an individual mass. The weight of the body is dictated by how much mass constitutes the physical body.

Hence gain/loss of weight is a Physical process.