This is the third year in succession that Edexcel Maths papers have been leaked to some extent before the exam. They didn't set a new paper in 2017 or 2018 and they're not going to do it this year either.
In the circumstances, the only way you can help your DCs is to report the details of any leaks you are aware of to Pearson (Edexcel). At the moment there are lots of rumours and hearsay of how many people had seen the paper, but a lot of that is probably exaggerated. If Pearson has the names of the people / Groups / Twitter accounts who shared the images and/or bought the papers, they'll be able to see how widespread or not it is and take appropriate action.
It seems to me that they very quickly identified the source of the leak (in East London) and will now be trying to plot out who saw it after that.
Although it's a worry, please try not to let your DCs get too stressed about it!