When my kids were small, the “experts” said that for pre-school age kids, you need to look at their nutrition over the course of an entire week, not just what they ate at any given meal. They go on one jag, and then another, depending on what they need. Expecting lunch to be a “square meal” every day is going to result in a lot of wasted food, especially if there is not an adult supervising, as at a family dinner table.
Pre-schoolers, heck! I eat that way at a ripe half-century of age. I'll go for days wanting nothing but protein and carbs, then go on a veggie binge for several days.