How do i tell young children their nan has died and gone to heaven?

I know i have asked a lot of morbid questions my nan died last year, i gave birth this year and my mother died today (this year) so i am managing but how do i tell my children that their nanny has died and gone to heaven with their nanny (2) its goin to be hard for them 2 deaths in just over a year and a half. My children are 6,5,3,5 months. I am sorry for all the morbid questions tonight. i guess i cant think properly at the moment and when should i tell the children as i am sending them to school tomorrow as they dont know just yet