The strange thing is, they could be accommodated on any of the 4 flights on Wednesday; two of those flights are available for €605.56 total for the 4 of them (one way)
They could travel tomorrow for €389.60 total for the 4 of them (one way) on FR
Due to the issues at AMS, KLM is only selling full-fare Y economy for the coming days, so a KLM ticket would cost a hefty €1,647.56 in economy (one way) for the 4 of them for most days this week.
As I can't read the article, what reason was given for charging them extra? Under EC261/2004, they are entitled to free rebooking.