Winston’s Wish provides support for children and young people following the death of a sibling, parent, or a person important to a child.  There are lots of resources on their website

Winston's Wish - giving hope to grieving children (winstonswish.org)

Kemp Hospice provide support for children and young people following and prior to the death of a sibling, parent, or a person important to a child.

Kemp Hospice - Support for the families of adults with life limiting illnesses and for bereaved families 

The Childhood Bereavement Network offer resources and support.

https://childhoodbereavementnetwork.org.uk/get-involved/resources/resources-families