Kids grieve differently but they can understand what's going on. Also telling her is a good way to avoid potential resentment towards you. She can understand why you're making the decision and is less likely to think you took her pet away. Explain how this is affecting you and it can help her know how to navigate the loss in a healthy manner. You can have a ceremony afterwards that she is in charge of. When my pets died we did this, it helped us as kids. Hope everything goes well for your family, sorry for your loss.
If you take care of other animals, sooner or later they will pass away.
This can be hard for children.
Reassure them that their sadness and tears were totally normal. If they have questions, let them know if they have any questions you are available now or anytime to answer them.