GRIEF and DENIAL are major topics in A Monster Calls. As an example think of Conor's attempts to make things appear normal and his rage whenever changes are discussed.
To start you can - from the Library's Advanced Search screen - change the field that contains "Any Field" to Subject and then in the first search field add grief (This is saying that everything you are searching will be about grief) then in the second search field add child* (the * indicates to the database that you want to use both child and children. Other terms to try include bereavement, loss, parental death, and parental loss.
Search result example: