From April 2016, You can carry children below 5 for free (without berth i.e. you won't get additional seat for your child). Quoting for official source
Children under five years of age will continue to be carried free (without berth).
However, if child is between 5 to 12 years, half price if you DO NOT want additional seat OR full price if you DO want additional seat.
full adult fare will be charged for children of age 5 years and under
12 years of age if for whom berth/seat (in reserved class) is sought
at the time of reservation. However, in case berth/seat is not sought
for the children of age 5 years and under 12 years of age at the time
of reservation, then half of the adult fare shall continue to be
charged subject to the minimum distance for charging.
It is a good idea to take some birth proof (like birth certificate) to prove that your kid is under 5 years.