You have made a significant mistake. We must not tell to the children that Carbon Dioxide (or CO2) is the "air" that a plant needs to breath. Plants breathe oxygen! In plants there are two parallel processes. That photosynthesis and respiration. Photosynthesis occurs in the light. Respiration predominates in the dark. There are plants that are dangerous to keep at home because it has the high level of respiration. Children need to know that. But most plants breathe much weaker animals.