Don't lie. You owe them the truth and it's a learning opportunity for them to grow emotionally before having to deal with a human death. Emotional Intelligence is just as important as being able to read.

Analogies can help but don't hide behind them. Greive together. Have a gesture, no matter how simple that encourages closure. Perhaps go through photos of the pet with them and ask which should be framed or printed or put on the fridge or something like that.

Talk to them about the emotions you are having, how those emotions feel and allow them to also have emotions, especially in the coming weeks when they miss them. But you don't have to lie about what you are feeling or make a huge deal about. Don't rush to fill the void too quickly. Comfort, yes, perhaps a new pet in due time. But not immediately as that cheapens the connection from being with an individual to being a role.

It sounds cruel for me to say I hope your kid has to endure many human deaths in their life but the alternative is either for them to have far too short of a life or for then to become unattached.

And I hope they will also be there for their friends when their friends have to deal with a loss. Kids model behaviour more then they do as they are taught, so it's an opportunity to model that kind of behavior and emotional support.