Muggles (such as Lily's mother and father) can go onto the platform, presumably the same way that wizards and witches are able, through the magical gateway.
And the scene reformed. Harry looked around: he was on platform nine
and three-quarters, and Snape stood beside him, slightly hunched, next
to a thin, sallow-faced, sour-looking woman who greatly resembled him.
Snape was staring at a family of four a short distance away. The two
girls stood a little apart from their parents. Lily seemed to be
pleading with her sister; Harry moved closer to listen.
‘… I’m sorry, Tuney, I’m sorry! Listen –’ She caught her sister’s hand
and held tight to it, even though Petunia tried to pull it away.
‘Maybe once I’m there – no, listen, Tuney! Maybe once I’m there, I’ll
be able to go to Professor Dumbledore and persuade him to change his
Harry Potter and the Deathly Hallows — CHAPTER THIRTY-THREE — The Prince’s Tale
Under normal circumstances (e.g. where the Muggle isn't accompanied by a young wizard or witch) and they notice someone using the entrance, their memories are simply erased
There are usually a number of plain-clothed Ministry of Magic
employees on hand to deal with any inconvenient Muggle memories that
may need altering at the start and end of each Hogwarts term.
Pottermore - King's Cross Station By J.K. Rowling