We don't generally have 3 full meals a day, as eating out that much is usually too much food. Definitely feed a pre-schooler from your plates unless the portions are very small or you are big eaters.
We generally have a big late breakfast, and then lunch out at about 2/3 pm and maybe snacks/chips/tapas in the evening, or we have the small lunch and then a meal out in the evening. So if we're staying B&B we only pay for one restaurant meal a day, or we buy bacon, eggs etc from the supermarket if self catering.
We also have BBQs and simple meals in the apartments such as ham, cheese, olives, bread, ready made pizza, tapas, barbecue, salads, crisps etc and we also stock up on water and alcohol in the supermarket to limit the amount of drinks we buy at restaurant prices. We will generally have a drink or two in a restaurant, but are just as happy to have a drink while reading on the balcony.
Once you get a feel for how much food a restaurant serves, you can tailor your order accordingly. When we were in Gozo, we found the portions so massive that a starter and a main to share was more than enough, even though we were on an activity holiday so quite hungry - first two days we left so much food as we were in a hotel with no kitchen facilities to warm up leftovers - if self catering I often take leftovers to eat the next day - like the time I got a meal that was five lamb chops,