OP,
At nearly 60 I'm closer to your Dads age than yours.
You need to tell him.
You can ask him to not make it worse by becoming aggressive, but, it's hard to convey the utter devastation and widespread grief him murdering you would bring.
Your father's life would be over, he would NEVER recover, your mother the same.
Life utterly changed.
Your children's lives would be blighted with a loss and grief that would haunt them for all of their days.
You are young, in your 20's.
It simply cannot be overstated how shocking his actions were.
The police and your father need telling.
YOU owe your children the chance of a future with their mum.
At my age I know of some children that have lost their mum through cancer and brain haemorrhages.
There are really no words for the grief of the children involved.
I have seen it touch my children, through them witnessing the grief of their lovely friends.
It is always with them.
Mums are completely irreplaceable.