Disclosure: I received complimentary products to facilitate this post. All opinions are my own.
Grief can be hard for kids to process. Whether it's the death of a pet or person close to them, the loss of a special object, or transitions that result in someone moving away, it can be hard for kids to process the feelings that come with loss, or moving through to a point where the pain isn't so hard anymore without feeling guilty for not feeling bad.
My Yellow Balloon is a powerful book for helping young kids (approximately ages four through nine) explore and express those feelings. When a little boy loses his best friend - the yellow balloon that accompanied him everywhere - he thinks he'll never be happy again. But gradually he gets to a place where he doesn't miss his balloon as much anymore.
It sounds trite, but right away my older daughter picked up on the theme, and made the parallel to when her best friend moved away this summer. She's made other friends, and although she misses this girl, she knows that it's ok to be happy with other friends.
The nice thing about the way this book is written is that it is so flexible. It's not necessarily just about death, but it could be. And it's also written in a way that won't contradict with any religious beliefs, but can provide a starting point to discuss what you believe about loss and death with your kids.