I’d need more context to give you a real answer but personally:

Do it like any “we need to talk” adult talks, keep it simple explain what happened, give age appropriate details, and tell them it’s ok to cry/be mad/be sad for a long time, etc. encourage them to express their feelings in that moment and then in recovery share with them when something reminded you of the pet or things related to the pet, it seems counter intuitive but they are having these same thoughts and feelings and by saying them out loud you’re saying it’s ok to feel this and we can grieve together.

Hope this helps