I have tried using Splenda but the result tasted like… Splenda. Not tasty if you ask me. It works, it makes the cider sweet, but it tastes like chemicals.
You might try lactose, that might be better. It’s not fermentable, but it’s also not very sweet. Add about 1 to 1.5 lbs per 5 gallons and see what it does for you. I have yet to try this in earnest. I’ve made an apple ale many times with lactose and I like the results, but I haven’t tried it yet with a straight cider. I think it would work.
Other than perhaps the lactose idea, honestly I really haven’t had a ton of success getting a naturally carbonated sweet cider. It either ends up sweet and uncarbonated, or more often, dry and overcarbonated (dang it!). You can try backsweetening along with sorbate and sulfite, but your results might turn out like mine – likely to continue fermenting to dryness. The yeast just doesn’t want to die!!
One thing that’s also helped keep my ciders with a little extra sweetness is to rack very often. Rack to secondary, tertiary, quaternary (is that even a word?!), every week or two, to get rid of as much yeast as possible during active fermentation. This helps the yeast to grow sluggish and prevent fermentation all the way down to 0.992. Because if you leave all the yeast in there, odds are it will hit 0.992 every time. I can keep gravity above 1.000 just by racking very often. But beware the bottle bomb, which is always a possibility if there’s any residual sugars in there. Sulfite and sorbate are your friends as well.
Another option might be to prime and carbonate (i.e., let the bottles sit for a month or so to carbonate naturally), then to kill the yeast with heat. Maybe run the bottles through the dishwasher heated cycle, that should kill 'em, yes? I haven’t tried this yet, but in theory, it should work.
Overall, I guess my primary suggestion would be to try the lactose along with the priming sugar, I think it might have the best shot at turning out well. Of course, if any of your friends are lactose intolerant, they might not appreciate this!
FYI – standard priming sugar for cider would be about 2/3 cup cane sugar per 5 gallons, somewhere around there. Or maybe 3/4 cup would turn out even better in a cider. I wouldn’t use that much for beer, but for cider it should work nicely, since cider should have no head on it. Add the priming sugar and lactose at the same time on bottling day. Dissolve and boil for a few minutes, cool, and add to the bottling bucket/carboy in bulk. Don’t try to add sugar to each individual bottle – do it in bulk. It’s a lot easier.
If you figure out something that works well, please let us know! After several years, I’m still trying to perfect cider carbonation. This year I got lazy and I drank all my hard cider flat/uncarbonated. It’s still good that way, and very traditional. Next year I’ll probably play around with lactose or something as stated above to try to get carbonation AND sweetness. Maybe it will work better than my past attempts, which have usually ended up being either flat or gushers.