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.
Related:
Dino-themed Terrariums
100 DIY Ideas for the 3-Day Weekend
Danielle's Charming, Classic Walk-Up
Good Food to Share 
The Big Book of Small, Cool Spaces 
Apartment Therapy Presents 
The Eight-Step Home Cure 
White Enamel Flatwa...
How To Cook Fluffy, Tasty Quinoa
Perfect Summer Cocktail Recipe: Honey Bourbon Lemonade
We have 4 boys and 4 stairs to the front door, these would look great!!
My four year old daughter is completely obsessed with all things dinosaurs. She would love these in her room!
These rock so hard.
I have these--a triceratops and a stegosaurus--in my son's room. He's 2 1/2 and can't resist playing with them sometimes, and the plants are quite hardy (luckily)! These are super cute and I'm thinking about buying more...
Mint!