Especially if they've got their faces smooshed against the window, poking their tongues out and shouting taunts.
A bit like this...
...but five more monkeys, like stacks on, all trying to get a look at that person who looks different.
Don't just sit there oblivious to it all.
Discipline them. If you believe in giving them a light smack, do so. If you discipline them in other ways, like taking their toy away from them, do that.
Teach them at an early age about diversity and acceptance.
Teach them about illness and disability. And about compassion.
Teach them that someone who looks different to them and that doesn't fit the beautiful mould isn't something to be ridiculed.
And don't just say that they're young children and don't understand their actions.