I think it is not on (don't stone me). If a employee with a baby/child works in a business/company that has a creche/daycare centre for employees on site, then that would be a good thing - mother could put her baby in there and visit it in her breaks/when it needed feeding (if b/f). 

Will the Mum be working full-time? 

I wouldn't like to work with a 3mth old baby in the same room. Iwouldn't like to take a baby to work with me, wouldn't be able to give either the baby or work my full attention.

I am assuming your employer has given permission for the mother to bring her baby into the office? How long is she able to bring the baby into work for?

Babies of around 4mths can start rolling around the floor etc.. how are they supposed to do that in an office... maybe a playpen? but thats not much life for a baby/infant..