Suppose someone from "Harry's world" married a Muggle, and they want to escort their child. Can the Muggle parent get through to the Hogwarts platform?
Here's Lily's parents on platform nine and three-quarters.
And the scene refomed. 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.
Lily glanced towards her parents, who were looking around the platform with an air of wholehearted enjoyment, drinking in the scene.
Harry Potter and the Deathly Hallows - pp.536-7 - Bloomsbury - Chapter 33, The Prince's Tale