For kids, a simpler definition―something like 'it's the process by which plants capture sunlight and use it to convert carbon dioxide and water into food'―will serve the purpose.
Photosynthesis is the process by which plants convert carbon dioxide into their food, by using the energy derived from the Sun.
At night, however, things are different as only carbon dioxide is released in the atmosphere.
It might be because of this, but many people believe that it's not good to keep plants in the bedroom at night.
Carbon dioxide (CO 2) is a chemical compound
In the first case, oxygen is released as a by-product, and in the second, carbon dioxide is released as a by-product.
In plants, photosynthesis only occurs in the presence of light, but respiration occurs day in and day out.