How to help your parents scan old negatives and slides
As your first step, order a Canon CanoScan 8800F scanner online. I was unable to find this item at any brick-and-mortar store in Seattle, the less expensive dedicated slide scanners which Costco carries have received terrible reviews, and at this point you're unlikely to find a reasonably-priced local service to scan negatives and slides for you.
Start by scanning negatives and slides that contain text: this is the quickest way to figure out if you have the source material upside-down.
In the provided PhotoStudio application, use "Color Negative Film" for color 35mm film negatives, "Monochrome Negative Film" for black-and-white 35mm film negatives, and the equivalent settings for slides: keep in mind that you will always have better results scanning an original negative or slide than you would from any print made from said negative or slide.
Select 2400dpi as the output resolution [Note: by way of comparison, if you were forced to proceed from prints rather than negatives/slides, then 600dpi would have been the maximum resolution which could have ever made sense, and 300dpi would probably have been more realistic.]
If you encounter square 35mm negatives (which were typically produced by old Kodak Instamatic cameras), then the provided PhotoStudio software will be unable to automatically discern which image is which. If you encounter such negatives you should still use the provided 35mm negative tray to secure the negatives, but have "thumbnail mode" off when requesting the preview, use "Rotate images 90 degrees left" after acquiring the preview to display the images horizontally, and then use the mouse and "Del" key to manually adjust/create/delete the cropping frames before pressing Scan.