Everyone's making good points, but there are a million and one ways to attack grass.
Honestly, with a cartoony style like this, I think areas of the same colour don't necessarily look too bad at all. Yes, you can break it up with patches of earth and variations in grass colour, and yes, where one shade meets another (or where grass meets another object, like a bush) you can add grass and leaf shapes to the edges to give more texture. And those things will help a lot. But you don't always have to fill every bit of the image with loads of texture.
You've got some hard shadows on your characters. Adding shadows to the plants in your grass might help too. Also, leaves have a crease from tip to stalk, so it looks more natural if you divide them vertically with shading, as opposed to horizontally like you do here.