Choose a subject for your ballad. This might be any of a wide range of things, from a child to mythical figures or even real people, if you want to make the ballad educational.
Write the beginning stanza of the ballad by introducing the subject in the situation. For example, if your subject is a creature, then set the stage for the story by telling about where the creature is using the first two lines of the ballad's first stanza. Use words that are appropriate for the age group.
Introduce the situation that the creature is presented with in the second two lines of the first stanza, rhyming the second and fourth lines. The actual length of each stanza should be enough to keep a child entertained, meaning that if it is too long and cumbersome, it will not keep the child interested. A good basic number of words for each stanza is approximately 25 to 50.
Write the remaining stanzas of the poem using the same structure as the first. Don't worry about how many stanzas you use. As long as the story is told from beginning to end, the number of stanzas don't matter. One of the most important aspects of a ballad is that it tell a story. For instance, in the first stanza, you introduce the subject of the ballad and the situation that she finds herself in. In the second, she may encounter an obstacle. In the third, she may overcome her obstacle. In the fourth stanza, the subject is seen living "happily ever after." Take note, however, that you can use as many stanzas in your ballad as you wish.