My 2 year old made us a party of 6 a couple of times regarding tip. On the plus side, he was free. He got his own (empty) plate and his own drink at each TS. You can fill his plate at buffet, or share from your own plate at other TS. The receipts shows his seat as an "Infant KM $0.00".
For what its worth, when they automatically added the 18% tip it resulted in the least amount of confusion in settling the tip with your room card.