Find a source of inspiration. Children usually respond very well to humor, especially if they are images that evoke a funny situation or scene. You can create content for your poem using an image that you consider fun or enjoyable and, from there, you can explay.
For example, if on the way to work you found an angry pigeon, you can use that pleasant encounter with the pigeon as a source of inspiration for your poem. Children can have fun when you retell the story of how you did to escape that dove on a busy street in the city.
You can also use some memories of your childhood or some of your interests as inspiration . Consider everything that made you laugh when you were young and everything that made you curious or intrigue. Doing this can help you get into a child's mind and remember what subjects made you laugh when you were little.
For example, maybe as a kid you had fun playing with plastic snakes. You may remember the moment when your brother tried to put the plastic snake in his nose and then you tried to help him get her out of there. This can inspire you to write a funny poem about plastic snakes in the nose.