Totally legal and that's what we did!! You can choose - either pay out of pocket to buy a meal for your 2 year old, or let him/her share with all of you or just your oldest. Ours were 4 and 2 (almost 5 and 3) when we went in the spring. They both eat like birds, so sharing one kids meal was PLENTY for both of them. At all the table services we ate at, they even gave DD her own chocolate milk free of charge (we planned to pay OOP for those). They usually got some sort of pasta dish and they would bring an extra plate for me to split it, no problem. Dont' think they ever finished a kids meal (except dessert, which we also split!)
AT counter service, most kids meals were an entree (chicken nuggets, burger, etc.) plus 2 sides - we got applesauce and grapes + dessert. They split the entree, each took a side - 1 likes each, and then split the dessert. I'd usually just scoop half of the dessert into the lid to share it. They'd also just share the drink (I'd pour half into cups we had) or one would just drink the water we brought in. They did finish the CS meals, but it was still plenty. You also have snack credits, so you could order something with one of those to supplement if needed.
Also keep in mind that at any buffets/all-you-can eat family styles, your 2 year old gets their own plate and can have whatever they want, no charge.
Hope this helps!