Children under 3 are always free at "admission" meals (buffets, family-style, dinner shows), regardless of whether or not you're on the dining plan.
Everywhere else -- they're generally assumed to be eating off your plates. While it's possible a TS restaurant might bring out a little something for them at no charge (especially if you have other kids eating with you), it's generally unwise to plan on that happening.
You can, of course, order extra out-of-pocket. It's generally unwise to "up-age" an under-3 child to get them on a dining plan (exception: on QSDP, if you're already planning on getting them an extra kids' meal each time).