So very sadly our children only have one Grandparent on each side. My father died in 2002 and my mother in law died 3 weeks before our eldest was born in 2007.
It’s always been very important to us that the children know about them and that we talk very openly about them etc.
I believe in God and therefore it was very easy for us to know that we were going to explain that they had died and gone up to heaven. When they were younger we said up in the stars and then gradually bought in the word Heaven.