Parents of Alfie and Harry McQuillan describe what happened when they discovered the overdose of morphine had been given.
One twin was given ten times the recommended dose while the other twin was given 17 times the normal dose.
Both died soon after the drug was given.
More top news
They will be joined by Duke Dumont and Disciples at this year's festival in Staffordshire.
There is no evidence the Westminster attacker Khalid Masood was associated with so-called Islamic State or al Qaeda, the Met Police said.
A cloudier start, but turning dry, sunny and warm.