Food Safe 3D printed Jello Molds
I'd recommend using ABS for food tools, as well as cleaning in the dishwasher. The heat from the dishwasher will kill whatever contaminants you might encounter and the ABS plastic has a high enough melting point to avoid being ruined. PLA will just melt in the dishwasher and some cookie cutters have details you won't be able to clean properly by hand without risking damage to the cutter, or loosening pieces that could break apart in the food unknowingly.

I'm no biologist or anything so I don't have any scientific evidence to support this claim. It's just the info I got when I asked this same kind of question to things4thinkers.com before ordering some cookie cutters from them. They're all 3D printed and the ones I got were ABS because I asked for them that way. So far I haven't noticed anything fishy about the cutters and they have held up in the dish washer fine.

I wouldn't do the foil thing personally. It seems like a lot of effort that will probably alter the mold you're trying to make. Or just end up being a pain when trying to cut cookies. Plus you still risk fragments of the foil being left behind in the food as well.

By the way, ABS plastic can be purchased from any number of manufacturers. Some may not be considered "food safe" where as others specifically state they are. Look at this "food safe" filament. Just make sure your printer can handle that filament, though if yours can handle ABS it should be able to handle this. Not an expert though.