Have kids view different breeds of given animals. (type into your browser: breeds of ________ images. Put the desired animal in the blank.) Pose the question, Where do such breeds come from? The answer in every case should be: Selective breeding. The process of selective breeding is briefly described in the following video.
The process begins with the observation of slight differences (variations) among the individuals in a population.
Selecting those with the variant(s) tending toward the desired end, e.g, cows producing the most milk, and having only those reproduce the next generation.
From the multiple members of the next generation, repeat the same selection and breeding procedure to get a third generation. Select and breed again, and so on.
Development of any particular breed has likely occurred over literally hundreds of generations and still continues being conducted by professional breeders. (Career opportunities)
The same applies to plants. Only with plants, variety is used place of breed. For example, type into your browser: varieties of tomatoes
Emphasize that each variety is a product of selective breeding as described above.