Before anything about how to say it: you are about to do one of the harder things grief asks of a parent, which is to hold someone else’s heartbreak while your own is breaking. You may not get to come apart the way you need to, because there is a smaller person watching your face to learn how frightened they should be. That is its own kind of grief — the kind you have to set down in order to do the thing in front of you. We want to name it, because no one else is going to.
On the saying itself, the thing most people who sit with grieving children come back to is plainness. Soft, true words tend to travel better than gentle-sounding ones that confuse: “died,” said kindly, usually lands more safely than “put to sleep” or “went away,” which can leave a young child frightened of sleep, or waiting for a return, or quietly wondering what they did to make the pet leave. You know your child and we don’t — but children can generally hold the truth told gently far better than they can hold a mystery.
Their grief will not look like yours, and that can be disorienting. A child might cry hard for ten minutes and then ask to go play, or seem fine for days and come apart at bedtime a week later, or ask the same blunt question again and again. None of that is them not caring, or you having explained it wrong. Children grieve in bursts, circling back to it as they grow into more of what it means. And letting them see that you’re sad too — not shattered in a way that scares them, but honestly sad — tends to teach them more than any careful speech can: that loving something means you grieve it, and that grief is allowed in this house.
You will probably worry afterward that you did it wrong — said too much, said too little, cried at the wrong moment, or didn’t cry and seemed cold. Almost every parent does. There is no version of this conversation that doesn’t hurt, because the hurt is the whole point of it: something they loved is gone, and you can’t fix that for them any more than you can fix it for yourself. What you can do, and what they will remember long after the words are forgotten, is not leave them alone inside it. That was the only thing the conversation was ever really for.