what is it like being part of the trial in their positions?

Presumably, this question refers to Scout and Jem, Atticus, Finch's children, and their relationship to the trial of Tom Robinson. Their father, Atticus, is the defense lawyer for Tom. The case is a high-profile, controversial one, and Atticus naturally does not want his children to be exposed to the mature content. However, Scout and Jem cannot resist and sneak into the courthouse to observe. 
Scout and Jem occupy a complex position in relation to this trial. They are children, innocent to much of the racism of their community. Their father is the defense lawyer so they are inclined to want him to win the case. However, they find that there is some animosity toward their father's role, as much of the community already find Tom Robinson guilty.
As the children watch the trial, they become outraged at the racism of the community and the assumptions people make about Tom. They are sympathetic to Tom's plight and feel the danger their father faces in defending him. Though they must lose their innocence by witnessing the trial, Scout and Jem also gain a greater understanding of justice and injustice and become aware of the underlying prejudices of their own community.