Good for you @Tretinker. After living in a flat in London, I lived in a terraced in the north of England. It had a fenced back garden but with a small greenhouse and a garden shed, and parking for our car, there was no room for the bantam chickens I had always wanted from childhood. We moved...
You will need to empty and refill the pond daily as it will get dirty!!
You can hose down places where poop gets too smelly. My ducks come up onto my patio and back steps and so I hose them down every evening. The effluent flows onto my asparagus bed and ornamental ginger plants. The latter...
10 metres x 10 metres is perfectly adequate space for 3 or 4 ducks.
Make sure it is predator proof -- foxes, owls and hawks would be a worry.
You can certainly use a kiddie pool.
Yes ducks will get water everywhere, but you can use deep litter in their duck house -- scoop out messy bedding...