Fine, kick out the parents. They have failed in their duty of care. He was still a child when he stabbed dozens of children at a party. You might be surprised at how casual the home office are in kicking out decent folk who have arrived from overseas. Why not kick out the ones who have raised child murderers or child rapists?
Yes it is "unfair" to "take it out" on the parents but there is a very serious problem here and it needs to be addressed before the UK literally becomes a foreign country to the indigenous population. These monstrous criminals might not care what happens to them if they get caught but they might think twice if their family suffers.