Hi OuiOui,
We too have a very small garden, it was split into two levels, patio on the bottom and grass on the top. When our triplets were a year old we decided that it would not be safe for them so this is what my husband did! We kept the two levels, but we did away with the patio and grass and put decking down, with three wooden steps up, but a gate across at the bottom to stop them climbing up. We also put a short fence to stop them falling from the top deck to the bottom! Then Harry built them a playhouse on the top deck, 8ftx6ft, with a stable type door, windows with shutters, using plastic sheets for glass. He painted it in Cuprinol paint, a sagey green and forget-me-knot blue, very pretty. Because of its size we were able to get vitually all their toys which are normally under the stairs out there, great, no more clutter indoors.
So all year round, even in the winter as long as they are wrapped up, they can get out, no mud to tread in. In the summer its great, we are all bare foot as the wood is so warm, and of course its great for pushing trains, etc along. The other bonus is its easy to keep clean, just sweep it up!
Harry built them a sand pit next to the playhouse, with a wooden lid, they had that until they were three, then got fed up with it, so we dismantled it, giving them more room now as they are getting bigger. Its great inside the playhouse, they have a kitchen area, and the boys have a workbench, plus a table and chairs, a bookcase, etc etc etc.
Hope this gives you some ideas, it has been brilliant for us, we still have flowers, we just put them in nice large containers, then we can move them around the garden to create a different look.