Trump is trying to humiliate China's current ruling elite through an overt intimidation of North Korea.
If North Korea capitulates, it will prove a loss of face for Xi Jinping, who would then find a freshly emboldened Jiang Zemin at his throat.
Elaine Chao(aka "Mrs. McConnell"): The Real Chinese Ambassador
The parallels between Saudi Arabia and China should terrify Xi Jinping: just as Team Trump backed a successful (for the moment) bloody coup in Saudi Arabia, he also overtly backs the anti-Xi faction.
Elaine Chao and Mitch McConnell have deep ties with Jiang Zemin's faction.
Xi Jinping: Tried to Assassinate Jiang Zemin
They are only rumors, but many of the "industrial accidents" in recent memory were directed either at Jiang Zemin himself (especially as he traveled by train through a particularly heavily-industrialized region) or his backers.
It resulted in counter-attacks, which spread all the way to the Chinese proxy-state of Thailand and helped metasticize the civil conflict, there.
North Korea: Intolerable Blackmail After Saudi Coup
Before the Saudi Coup, the world was a less dangerous place.
Now, Xi can not tolerate a DPRK humiliation: it will redound horribly on him and his clan, and the Zemin faction will, invigorated by Trump's success in Saudi Arabia, surely press to remove Xi's clan from power.
Solution: Take Hostages
The American media has given zero attention to three black UCLA basketball players "arrested for shoplifting". Regardless of whether or not three elite athletes with NBA prospects would really shoplift for a laugh, the Chinese government sought hostages that, they believed, would maximize the pain on Team Trump during the DPRK negotiations.
US Media: Blackout on Hostages
Until Trump has finished his China trip, the American hostage situation remains out of the news-cycle. In fact, the day Trump arrived, the UCLA athletes were mysteriously set-free on bail... but not free, yet.
Chinese Plan: Use Black Male Athletes vs Trump
The Chinese intend to play the race card against Trump, to make him seem unsympathetic to the black community while pushing for a asinine war with North Korea.
It is entirely plausible that Xi could have coordinated this hostage-taking with global actors seeking to corner Trump. That is an unprovable hypothesis.