What to plant in sheep poaddock?

Our paddock is little (too small) but it is still something.
The sheep and the chicken would need some better shade and I want to plant some trees there.
I thought on olive trees, maybe mixed with tagasaste (which we would take out later when the olive trees are big).
I know that we would have to protect each tree, and therefore in the beginning there would not be any shade for the animals. (Now we have the kids trampoline in there as shade).

What is the climate like there (water, minimum temp, etc)??

Up to minus 5°C in winter nights. We are cool climate, but we have plenty of sun, we're at 1000 m. I'm not sure but I think it is zone 8. We have an average rainfall of about 1000 mm, but the last two years it is extremely wet. But we are in Australia and we always have wet and drought.