I booked a ticket for my family online, but when I was filling the form I forgot to give details of my niece - he is less than 3 year old. Is it necessary to provide the details, and if yes, than kindly tell me where it is possible after booking and how. If this is not possible, will we run into any problem?
Children under the age of 6 are carried without any charge on Indian Railways. Though, it is always better to give details of children while booking a ticket, you will not run into any problem while traveling.
You will not be able to add the name of the child after booking the ticket. Changing the name of the passengers is allowed in certain cases with a payment only at the ticketing counters (I am not aware of the exact rules behind that).