"Also, it's not the OP's role to provide outdoor play facilities for another person's child; if the "poor child (is) looking sadly over the fence" maybe its CF parents can take the hint and fork out for their own toys instead of waiting for OP and her family to leave their home and sneaking their kid over the fence."
My parents neighbours used to help themselves to my daughter's toddler-sized plastic garden slide all the time, apparently. When we weren't at my parents, the slide was stored behind their woodshed, and the husband just used to lean over and grab hold of it, lifting it over the fence so that their toddler could play on it. Until the day I was the one who watched him do so. I may have risked my own life and limb to stick my head over the fence and demand to know what the hell they thought they were doing - and to return the slide which I'd bought, for my child's use, immediately. They rather shamefacedly did so, and then allowed their toddler to scream, despite a vast array of garden toys already littering their space, for a while in temper that "her" slide had been taken away by "the nasty lady next door".
What they didn't take into consideration is that, unlike some people, I don't believe in giving small children everything they demand, when they demand it. So I cheerfully sat out in the garden with a book and ignored the noise. It stopped eventually. And the slide (which the parents had obviously seen my child using from a window overlooking my parents garden, and felt entitled to its use for their child) was moved firmly out of their reach. Within a few days, my mother very pointedly moved everything else out of reach of that fence between the gardens. Just in case.
The wife, it later turned out, was lovely. But the husband was an arrogant bully who floundered when stood up to (which I did a few times, because he didn't frighten me - I'd lived through worse than he could dish out to me). Sadly, their children followed in their father's footsteps.
Things like this go on more often than we realise, though, I think. When we had our own garden, I insisted on a shed large enough to keep everything under lock-and-key - especially as my parents fence is 8' and ours is half that size between us and our neighbours (who had small children and grandchildren living there/visiting every weekend during the summer! They're teens now, so unlikely to want to borrow a toddler sized slide, or paddling pool - which is my dog's, as it happens...).