This is left vague in the books, but I can’t imagine parents sending their children off to a school they know nothing about. Do you think they’re well-informed, or told just a bare minimum, or are they outright lied to?
Muggle parents of wizards are told of magic and about Hogwarts.
Muggle parents of wizard children are told about Hogwarts when their child is accepted, someone comes to explain to them that their child is a wizard and will be accepted into a school for wizards.
“And will it really come by owl?’ Lily whispered.
‘Normally,’ said Snape. ‘But you’re Muggle-born, so someone from the school will have to come and explain to your parents.”
- Harry Potter and the Deathly Hallows, Chapter 33 (The Prince’s Tale)
They know that their child is magical, and are also allowed into at least some places in the wizarding world - Hermione’s parents go to Diagon Alley with her to buy Hogwarts supplies.
“They each grabbed a copy of Break with a Banshee, and sneaked up the line to where the rest of the Weasleys were standing with Mr and Mrs Granger.”
- Harry Potter and the Chamber of Secrets, Chapter 4 (At Flourish and Blotts)
They’re allowed into Diagon Alley despite being Muggles because they have a wizard in the family.
Exactly much they know likely depends on what their child tells.
Muggle parents are allowed to know all about Hogwarts and magic - there don’t seem to be any restrictions on how much they’re allowed to know. However, the amount of knowledge each Muggle parent would have likely highly depends on how much their children tell them. Hermione seems to have told her parents quite a lot about the wizarding world, since she’d told them a lot about Harry and Ron, she most likely told them a lot about other aspects of the wizarding world as well.
“I’ve also modified my parents’ memories so that they’re convinced they’re really called Wendell and Monica Wilkins, and that their life’s ambition is to move to Australia, which they have now done. That’s to make it more difficult for Voldemort to track them down and interrogate them about me – or you, because unfortunately, I’ve told them quite a bit about you.”
- Harry Potter and the Deathly Hallows, Chapter 6 (The Ghoul in Pyjamas)
It’s also possible for the wizard child to not tell their parents everything that goes on at Hogwarts. Dean didn’t tell his parents about Cedric’s death.
“My parents are Muggles, mate,’ said Dean, shrugging. ‘They don’t know nothing about no deaths at Hogwarts, because I’m not stupid enough to tell them.”
- Harry Potter and the Order of the Phoenix, Chapter 11 (The Sorting Hat’s New Song)
So, it’s possible that wizard children can hide information from their parents.
At the end of Deathly Hallows we see Snape's childhood memories. One of the memories consists of him telling Lily that someone comes to explain things to the parents of Muggle-borns:
"And will it really come by owl?" Lily whispered.
"Normally," said Snape. "But you're Muggle-born, so someone from the school will have to come and explain to your parents."
This tells us that someone comes to explain things, so the parents must know some amount of information, but it doesn't quite tell us the extent of what the parents are told.