My mother in law died when my son was 3 1/2 years old. We told him straight out that she had died after being ill. He was very sad, as you’d expect, but I think the sense of loss only started coming out a few years later. He still mentions his grandma and says he misses her so I don’t think they get over it as some people have suggested.
We’ve only just got around to telling him over the last few months that the graveyard is where grandma was buried, as we weren’t sure if he was ready to hear that at the time.
It was a hard thing to do, best of luck to you..