The Foundation highly recommends “Little Parachutes”

“We offer books which tackle the difficult subjects of death, bereavement and loss. Grieving children may find comfort or answers to challenging questions in the pages of these books. When a loved one passes away, it is important to find a suitable picture book that matches the personality of the child and the situation. The death of a grandparent, for example, may raise different questions than the death of a younger person. A sudden and unexpected death may evoke different responses in a child than a bereavement that has come from a longer illness. We have provided lots of books in this section to help you find one that works for your child and situation.

Website: https://www.littleparachutes.com/category/issues/death-bereavement/

We also highly recommend the work of The Child Bereavement Charity
https://childbereavementuk.org/for-families/support-for-bereaved-children/
https://childbereavementuk.org/for-families/support/