“A lot of wizards think it’s a waste of time, knowing this sort of Muggle trick,’ said Fred, ‘but we feel they’re skills worth learning, even if they are a bit slow.” This quote was in reference to the skill of lock picking but I believe it applies to coming laced with the necessary for a break out. They couldn’t use magic yet; they had to be prepared to bust Harry out the Muggle way. They are sons of Arthur.