A sprinkler will work quite well. We have 5 indoor cats (well 4 and an old cat who can’t get out of the garden). I’d sometimes put blame down at the ‘servants’ for not having fresh litter. We’ve also always ensured the cats do have somewhere to go in the garden. Our old cat, who can’t see very well, pushes behind some plants for some soil to have a crap if she can’t get in the house – she’s never crapped or peed on the lawn.
With all the cat’s we’ve had, most have come in for a poop – out indoor cat’s have a large cat run in the garden, with assess to shingle and soil to poop in. They still come in for a crap and a pee.
If you fancy a bit of fun, a super soaker is quite good – we’ve used them in the past with nightmare Tom cats – you don’t even need to hit them, but the surprise is often enough to stop them coming back.
Cats love freshly dug soil – maybe use bark. Despite many neighbour’s having cats, we don’t see any sh1tting on the lawns though. Might sound daft, but how about somewhere in the garden out of the way that’s got some fresh soil – away from the kids – behind a shed, end of garden ?. I know you shouldn’t have to, but the cats will go there.