Questions about Muggle parents getting onto Platform 9¾ are addressed Who gets Muggleborn students onto Platform 9¾, where it's verified that Muggle parents and siblings can indeed get onto the platform, so are able to cross that magical barrier (DH). I cannot find a canon reference to any known Muggle parents visiting Hogwarts. It seems unfathomable that any parent would send their child(ren) off to a boarding school a magical one no less without reassurance they would have access to their child(ren) as needed. A few things:
In Goblet of Fire, Hermione explains that Muggles cannot see Hogwarts:
‘[Hogwarts is] bewitched,’ said Hermione. ‘If a Muggle looks at it, all they see is a mouldering old ruin with a sign over the entrance saying DANGER, DO NOT ENTER, UNSAFE.’
Goblet of Fire page 148 Bloomsbury chapter 11, Aboard the Hogwarts Express
Does this mean Muggle parents simply cannot see the outside of Hogwarts, or does it mean Muggle parents, if somehow led inside Hogwarts, wouldn't be able to see anything or the interior of Hogwarts would appear as a ruin?
It's not against the rules for Hogwarts parents to visit their child(ren). Arthur and Molly Weasley make quite a few appearances at Hogwarts (CoS, GoF, HBP). Various sets of parents show up to collect students after Dumbledore is killed, such as the Patil twins, Zacharias Smith, and Seamus Finnigan (HBP). Of course Lucius Malfoy makes his appearances at Hogwarts (CoS), and Slytherin student Graham Montague's parents come to Hogwarts after Montague Apparates out from the broken Vanishing Cabinet where he had been trapped by Fred and George Weasley (OotP).
Finally, Petunia managed to send a letter to Dumbledore when she was still a child (DH), so the barrier between the Muggle World and Hogwarts must have some flexibility.
Or am I wrong and all the parents of Muggleborn students are just plain out of luck when it comes to visiting their child during the school year? Do Muggle parents visit their Muggleborn children at Hogwarts, or are they totally excluded?