Ideas for mostly shaded kids area

Hello, I'm fairly new to gardening (and in the UK) and I've made the back of our garden into a play space for my son – I'd like to plant something wild that will spread and create ground cover, that doesn't mind the shade or being trampled.

Does such a thing exist? I've posted some photos of the space.

We're south facing and in the midlands.

Really appreciate any and all ideas!