Dining Plan is only available as part of a package which includes room and Park tickets. All persons on the package must purchase the same type of tickets for the same number of days.
If you show your child as under three there will be no tickets needed and not included in the DP. If you state the child is between three and nine the child would be charged for and included on the dining plan and you would also have to purchase a child's ticket for the same number of days and same type (PH, etc) as the adult tickets.
You could do it and save the ticket for a future trip, but then again you could over the years misplace the ticket.