We have taken our 5-year-old since he was 2 and are currently starting to take out 1 year olds regularly. My oldest has been getting his own off the kids meal since he was 2. The check meals are small and full of healthy snack foods, so that's one option, or a full size meal for that matter, that you can then save some of for later. There are toddler meals at counter service restaurants, too. This is usually mac n cheese, and it's a very small portion. My 1-year-olds each finish off one of those, so don't count on the toddler meals to be enough for an older toddler/preschooler.
At table service, if it's a buffet style, make sure the get the reservation for the right number, but your child can eat off your plate. If they're individual dishes, it depends on whether you're willing to eat something your kid would want or if you would prefer to spend the money on a kids meal and enjoy your own.