We had the same issue with my goddaughter, now six and a half.
Two rabbits, one hamster & two cats [one drowned], karked it over the course of eighteen months.
Not being superstitious types, her parents talked about pets having shorter life spans, not suffering and their spirit living on if they are remembered.
So far so good, but she is incredibly bright for her age and more difficult questions are sure to follow soon.
What disturbs me though, is that my friends garden is starting to resemble a pet cemetery. We all know how that book ended. 😯