The Weasleys along with Harry and Hermione could have Side-Along Apparated to the Quidditch World Cup, There are four licenced and professional practitioners (Arthur, Bill, Charlie, Percy) in Weasley's.
Why did they use a less comfortable way of Portkeys to the Quidditch World Cup?
“Some Apparate, of course, but we have to set up safe points for them to appear, well away from Muggles. I believe there's a handy wood they're using as the Apparition point.”
- Harry Potter and The Goblet of Fire, Chapter 6-The Portkey
We can see that underage wizards can Side-Along Apparate without any mess with Ministry of Magic from Harry's Apparition with Dumbledore
“You have not, of course, passed your Apparition Test,” he said.
“No,” said Harry.
“I thought you had to be seventeen?”
“You do,” said Dumbledore.
“So you will need to hold on to my arm very tightly. My left, if you don’t mind — as you have noticed, my wand arm is a little fragile at the moment.” Harry gripped Dumbledore’s proffered forearm.
“Very good,” said Dumbledore. “Well, here we go.”
- Harry Potter and the Half-Blood Prince, Chapter 4-Horace Slughorn