A relative may often accompany someone to an appointment, but whether the companion can be present during questioning is decided by the office, and a companion is generally not permitted to answer for the applicant. Confirm this in advance rather than at the door.
Sort out each person's own steps
If the relative needing assistance is a dependent rather than the principal applicant, remember that TD status for a spouse or an unmarried child under 21 has its own nationality-specific steps and does not authorise work of any kind, and remote or unpaid activities require individual review. A child who is a United States citizen does not need TD at all. Check who actually has to appear before arranging support for everyone.
Map the household one person at a time before deciding who needs support on the day. For each member, record three things: the status being sought or already held, whether that person must personally appear, and what the status does and does not permit afterwards. TD dependants may study, but the status authorises no employment, and an unmarried child ages out of eligibility at twenty-one regardless of anything agreed for the parent.
Hypothetical example: an agronomist arranges an interpreter and an attendant for a parent who, on checking, is not part of the application at all, while the spouse who does have to appear has no arrangement in place. A companion permitted into the room is there to assist, not to supply answers, and questions put to the applicant are expected to be answered by the applicant.