Have kids note the distinction between the the complex foods on our dinner plate and the simple molecules, e.g. glucose, amino acids, etc., required by our cells. The takeaway: Food must be broken down both mechanically and chemically (by enzymes) by our digestive systems.
The following video gives the major parts (organs) of the digestive system and the job of each part. Note the role of enzymes that are produced and secreted by cells of specific glands/organs associated with the digestive system. (Focus on only the first 3.5 minutes of the video. Latter portions get into details and complexites beyond the scope of this course.)
Don’t let kids loose sight of the fact that what we eat must contain the basic ingredients: vitamins, minerals, protein to supply amino acids, and carohydrates for energy. For example, the digestive system cannot make amino acids from carbohydrate (starch, sugars). There is no nitrogen, which is essential for each amino acid, in carbohydrate. There must be protein in the diet.