I may have understated their needs somewhat. Most of what they do is browsing and document editing, but a few key use cases make a real computer necessary (or at least highly desirable):

- Document scanning

- Label printing (my mom buys/sells stuff on eBay)

- My dad still works and writes proposals/manages invoices/does complex taxes

At a minimum, they need a full desktop environment. Most of these things have decent 1:1 Linux alternatives, but one or two might necessitate a single-purpose Windows VM when all else fails.

Two pretty decent used micro PCs will also cost less than a single iPad.