This is an easy task for a beginner. This is based on a simply question and answer technique. A student composes a short song, maybe two questions and answers. You can also make lyrics to the melody.
If you make lyrics also, note that you can do them first together in a piano lesson. Many times children have ideas to lyrics, but they find it difficult to make a poem that rhymes. So start asking questions. Who is the main character in this song? What happens and where it happens? What happens next? Then help your student to make a little poem. I recommend you to use rhymes, because then the poem is easier to compose.
In my example I use an easy accompaniment. Of course you can vary this as much as you want. Then let your student add klusters, glissandos etc between the frases.
What a fun little piece it will be!