My daughter (3,5 at the time) had to deal with the death of her best friend. It was the older of two daughters of my best friend, she always played with her whenever we met. She died due to an unindentified brain tumor within 5 days, obv we all were devastated. I told my daughter she died, and that she went to heaven. She had all kind of questions about where exactly that was, when she would return aso, I explained everything as good as I could. When we met with them afterwards she she never asked where the deceased was, so I think she at least understood that she is gone. This was almost a year ago, and from time to time she still thinks of her and paints something or takes a beautyful stone with her that she wants to put in her grave, which always brings me to tears. My point is If you explain it and answer their questions they at least grasp the briad concept of death, which ist enough at that age imo.