Does Platform 9 3/4 have some sort of Magical Blood Detection Charm?
For example, let's say that a Muggle saw a Hogwarts student pass through Platform 9 3/4 as though the wall was intangible, what would happen if the Muggle attempted to do the same? Would a charm exist to block them off since they don't have magical genes?
Does the same apply to Squibs? Or since they have wizard genes, but just repressed and not active, they can pass through the Platform?
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 mind!’
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.
