How can one improve one's good opinion of Allah despite the existence of the torment of the grave for children, such as the Messenger's daughter (peace and blessings of Allah be upon him), and why are believers included in graves while they do what Allah has commanded?

Having a good opinion of Allah is a religious obligation. Among the things that help achieve this is reading books on encouragement and the virtues of deeds. There is no proven evidence for the punishment of children in the grave. The daughters of the Messenger of Allah, peace and blessings be upon him, passed away after reaching adulthood and marriage, and there is no evidence of their punishment in their graves.