Fruit offers the same nutrients as vegetables. My child often drinks a glass of milk and leaves the table. Milk is an excellent food, but not perfect - no one food group can supply the nutritional benefit of a diet that includes all four food groups. A glass of milk can satisfy the appetite of a busy preschooler as well as filling up his stomach. I've heard that many preschoolers don't get enough.
Iron deficiency is a problem for some preschoolers.
Slow weight gain, loss of appetite, fatigue, irritability and short attention span can point to iron deficiency but most preschoolers can be like this at times. Serve iron rich foods often. Whole grain and enriched breads, cereals and pastas, meat, fish and poultry are major sources of iron. Blackstrap molasses and raisins added to muffins, cookies or baked beans will give them an iron rich boost. And last but not least, the vitamin C in fruit and vegetables will help the iron be absorbed and used.