Consider your target audience. Will the song be for toddlers, preschoolers, elementary-aged children or older kids?
Determine the purpose of the song. Do you want to educate your audience about something? Do you want to entertain and make them laugh? Do you want them to be involved in the song as well?
Decide whether you want to start with the lyrics or the melody. Either is okay. At first, try to start with what flows the easiest. You might even be able to come up with both at the same time.
Keep your lyrics simple, especially if you're targeting a younger audience.
Write about concepts that are relevant to your target age group. For instance, preschoolers can relate to ideas like learning to share, dressing oneself and simple emotions, whereas older kids would be bored with these topics.
Use simple melodic structure. Most popular children's songs have from one to three chords. Also, make sure that the notes are suitable to kids' voices. Usually a range from middle C to high C is a good place to start.
Write your song down and record it if you wish. Writing and recording will help you to remember your ideas during the songwriting process.
Practice performing your song (with or without accompaniment). Make sure you know it well and can present it easily.
Perform your song for children (groups or individuals). Children sometimes give very honest, helpful feedback that can assist you in refining your song and making it even better.