The whole misleading tweet comes from The Independent running a somewhat misleading headline in 2016. But it would seem someone in the Trumpworld dug this up and Donald Jnr ran with it.
Dumb, but lots of dimwitted Trump supporters will never bother going further than the tweet.
Update: I see from Catallaxy (where CL is running with the story - of course, selective quoting and exaggeration is his rhetorical speciality) that the source of this is from Gateway Pundit.
And in checking on what exactly Khan said in September 2016, it is clear that many English papers ran with "part and parcel" but without putting up the full sentence. Even when you go to the Breitbart version of the story, they don't seem to have the full sentence, and their longest quote goes with the unremarkable:
“It is a reality I’m afraid that London, New York, other major cities around the world have got to be prepared for these sorts of things,” he said, the Evening Standard reports.
“That means being vigilant, having a police force that is in touch with communities, it means the security services being ready, but also it means exchanging ideas and best practice,” he added.