Encontre lo siguiente en el Oxford Dictionary of Law, espero te sirva (es bien largo - las leyes se refieren a leyes de Inglaterra):
ad litem: [Latin] For the suit. a guardian
ad litem will be appointed by the court to protect a minor's interests in proceedings affecting his interests (such as adoption, wardship, or care proceedings). Since the Children Act 1989 came into force the role of guardians has increased and they must ensure that the options open to the court are fully investigated. However, if a child is deemed capable of instructing a solicitor on his own behalf, he may do so even if this conflicts with the interests of the guardian. A grant
ad litem is the appointment by the court of a person to act on behalf of an estate in court proceedings, when the estate's proper representatives are unable to unwilling to act. For example, the Official Solicitor may be appointed administrator
ad litem when a person wishes to claim under the Inheritance (Provision for Family and Dependants) Act 1975 but the personal representatives are not willing to act. or nobody is entitled to a grant, or the only person entitled to a grant is the litigant himself.
Espero que esto te sirva. Mi opinion respecto a la traduccion de este termino es "abogado/agente representante" pero tal vez otras personas en el foro tengan otra opinion, a veces los terminos en Latin se ponen en Latin, no en el idioma a ser traducido; espero que recibas mas ayuda de otras personas mas expertas del foro. Buena suerte.
Saludos
Isabel