Does anyone know anything about gardening?

Not sure if this is more diy but...

My OH is looking at picking up a 2nd hand Wendy house similar to this

240x170cm platform is 50cm high

Our garden has a slight slope and in the planed area its maybe a foot between the highest / lowest points. It is also heavy clay, although one side will sit on quite good earth. The garden is a thoroughfare for cats and foxes.

My questions are these:

1. I should put something under the feet right? Both to prevent rotting and sinking. Would this be a job for Eco bases? It feels like it.
   
2. Would you add feet to make them more stable with a larger surface area? Or will this just give a surface to collect water to help them rot?
   
3. What should go under it? Both in terms of plants and base. As fun as the kids playing under maybe due to the risk of animals, I'm inclined to add plants to discourage kids going under - am I right, or being silly? Plants I'm thinking Cyclamen, ferns, and something Junglely.
   
4. Any ideas for a "light" living roof? Ie shit growing on rather than full shabang
   
   

Cheers