No, It is not always included in the ticket. Please let us know during the time of booking if you want it too.
No, the railway ticket is always optional for the passenger (when offered with booking). It does not effect the ticket price.
It will actually depend on the booked ticket type and the airline. It is not always fixed.
Yes, we can make a child request to the airline along with the ticket booking. It will cost sometimes but solely depend on the airline.