Not only do plants improve air quality, having a child care for a plant is a useful way to teach responsibility. Provided your child is old enough not to eat or touch them, easy-to-care-for succulents are a good place to start. Better yet, put them inside a dinosaur planter. Etsy seller Plaid Pigeon has a selection of dinosaurs (repurposed toys) to choose from as well as a few Safari animals like a lion and rhino. Each planter ($18) comes with a succulent which will need watering about every two weeks.
Find them at Plaid Pigeon.