Kane was forced off just five minutes in with an ankle injury before Son scored twice to ram the taunts of Millwall’s fans down their throats.
Christian Eriksen kickstarted this comfortable win for Spurs against the League One visitors with a fine volley from just inside the box on 31 minutes.
South Korean Son, who was racially abused by the travelling fans, then scored either side of half-time to silence the jeers and kill off
Millwall’s hopes of claiming a fourth FA Cup Premier League scalp.
Dele Alli tapped in a fourth before Vincent Janssen swept in his first goal from open play five minutes after coming on as sub to add the gloss.
More Wembley practice for Spurs
Spurs will get some much-needed and extra practice in at Wembley if they are to win the FA Cup.
This victory earned them at least one more game at the national stadium, in the shape of the semi-final, and potentially two.
That could be useful with Spurs, as it stands, due to spend the whole of next season there while the new stadium is being built.
Spurs only won one out of their four European games there this season but now have the opportunity to make Wembley feel much more like home.
Sweats for Harry Kane
Spurs are facing a sweat over star man Harry Kane after seeing him forced off with an ankle injury once again. Kane signalled straight away after getting his right ankle caught under Jake Cooper’s body as the Millwall defender slid in and tried to block a Kane shot.
The fact he had to go off so soon and was in no shape to even try and play on made it an instant concern. So was the fact it was the same ankle Kane injured earlier this season, an injury that kept him out for seven weeks.
Son silences jeers
Heung-Min Son produced the perfect response after Millwall fans targeted him with racist abuse.
The away fans mocked South Korean Son by chanting “DVD” and “three for a fiver” every time he touched the ball in the first half.
But they were silenced when he curled a superb second goal past a helpless Tom King to make it 2-0. Then he rubbed Millwall fans’ noses in it with a brilliantly taken, second half volley.
The abuse was clearly noticed by Spurs sub Kyle Walker who aimed what appeared to be a ‘have that’ gestured towards the Millwall end while celebrations were ongoing.
The chants did die down after Son’s goal but it might not be the last we hear of the matter.
Vincent Janssen up and running at last
Finally, at the 30th attempt, Vincent Janssen has a goal from open play for Spurs. It was pretty clear where he was in Mauricio Pochettino’s thoughts when Harry Kane went off injured and midfielder
Christian Eriksen went on instead of him. Only when it was 4-0 did Pochettino think it was safe to send him on. He was rewarded when Janssen swept in Spurs’ fifth goal in the 79th minute.
And you could tell how big a goal it was as all of Spurs’ outfield players rushed to celebrate with him and it got the biggest cheer of the afternoon.
Millwall’s brilliant run to the quarter finals came to an end on a tough afternoon for the League One side.
At the back they were overworked as they conceded possession and tried to hit Spurs on the back.
And despite the best efforts of Jake Cooper and Byron Webster at the heart of their defence and goalkeeper Tom King it was always going to be a tough ask trying to contain a Spurs side handed the initiative.
Up front, Steve Morison and Lee Gregory certainly worked hard but little joy as they spent most of the afternoon chasing lost causes, with little support to back them up.