Here are a few ideas which might help:
FindMyPast has the details of the baptisms and, where relevant, burials of this family. They can be searched under ‘British India’. At least three other children were born in India and Sarah died on 17 Nov 1851 at Peshawar having given birth on 8 Nov.
Joseph was Band Master to the 61st Regiment of Foot and a google search will bring up that regiment’s history and where they served. It was a British regiment not a regiment of the East India Company Army.
FIBIS (Families in British India Society) website and wiki will provide much background information to life in India.
The National Archives at Kew may have a service record for Band Master Magrath. His marriage would be recorded on that if the service record still exists.
Have fun exploring!