Kids under 3 are free at buffets and can eat off your plate at other locations. Just be careful if you decide to order something for them and pay out of pocket that you are very clear with the CM--otherwise they may try and deduct one of your credits for their meal.
At TS restaurants, we told the server and then checked the bill before we left. At CS, we placed 2 seperate orders (one with credits, one for cash). At the food courts, I put the food into seperate piles and had the CM ring each up as an individual order.