For a reserved seat, if you have a child under 5 years old who needs his/her own reserved seat, then you need to purchase a "CHILD (6-11 YEARS OLD)" seat for him/her.You don't need to purchase a ticket for 5 years old and under if you are going to put him/her on your lap for the whole train ride.For the unreserved seat, children under the age of 5 can travel free of charge.Please visit the official JR page at http://www.jreast.co.jp/e/ticket/types.html for further information.