The Client by John Grisham

A US State Senator is dead. Only Mark Sway knows where the body is hidden.
And he's eleven years old.
The FBI want him to tell them where it is, regardless of the risk to the boy and his family.
The killer wants to silence him permanently.
Reggie Love has only been practising law for five years, but she agrees to represent Mark pro bono, knowing she's his best hope for survival. Against the twin threats of the cold-hearted American state and the schemes of a cold-blooded killer, Reggie must fight the case of her life.
Or it might be the last case of her life.