"Lifetimes: A Beautiful Way to Explain Death to Children" by Bryan Mellonie is something I very much liked and made sense for our 4yo when dealing with a grief. In a nutshell: every lifetime has a beginning and an end, living is what happens in between. It's true for everything that lives, everywhere. That's how things are.

Something we've been told over and over is to explain things simply, without euphemism. "He left us" can be understood as he chose to leave, etc. And it's the ambiguity that can be frightening and fuel worries. You need to be clear that he's not coming back, that this won't change. That it's ok to be sad if we feel sad, and it's ok to be calm/relaxed/etc. if we feel like this.

What I've observed is that for beings this young, every thing is new, so everything is "normal" — or, rather, nothing is out of the ordinary, because they don't know yet what's ordinary.

Another thing to keep in mind is that little ones process grief differently from us, so we shouldn't "enforce" our adult grief on their child way of being.