The dining plan is tied to your park ticket.
Children 2 and under don’t qualify for a dining plan, as they are not required to buy a ticket. They have to share meals with you, or you pay out of pocket, as we did last trip with our 2yo twins.
At 3 years, they have to have a ticket into the park, and thus, they can purchase a meal plan. I believe it’s around $10 a day, but can’t remember exactly.