Nope, they are too young to purchase the dining plan (they'd have to have a park ticket so it wouldn't be worth it to try and put them on the dining plan when you consider the cost of the park ticket).
It would be the just same for anyone whose not on the dining plan. The under 3 year old can eat anything off of your plate. If you eat at an all you can eat place like a buffet, pre-plated or family style place, then they can eat free all they want.
If you order them anything off of a menu, then you must pay for it.
There is SO much food on the dining plan, I don't think you'll have a problem feeding your toddler.