Could a Muggle access the Room of Requirement?
‘Because [the Room of Requirement] is a room that a person can only enter,’ said Dobby seriously, ‘when they have real need of it. Sometimes it is there, and sometimes it is not, but when it appears, it is always equipped for the seeker’s needs. Dobby has used it, sir,’ said the elf, dropping his voice and looking guilty, ‘when Winky has been very drunk; he has hidden her in the Room of Requirement and he has found antidotes to Butterbeer there, and a nice elf-sized bed to settle her on while she sleeps it off, sir ... and Dobby knows Mr Filch has found extra cleaning materials there when he has run short, sir, and –’
Order of the Phoenix - page 343 - Bloomsbury - chapter 18, Dumbledore's Army
The Room of Requirement is clearly an incredibly magical part of the Hogwarts castle; its powers even broach the possibility of sentience. To access the Room of Requirement, whoever is seeking it must walk past its wall three times and concentrate very hard on his or her need. It doesn't seem as if any magic is involved on the seeker's part, though. If Mr Filch -- a Squib who has no magical abilities -- can access the Room of Requirement, would that suggest it would be possible for a Muggle (such as a visiting parent) to access the room as well? I haven't been able to find any examples in canon where wizarding magic is necessary to enter the Room of Requirement.