Upvote:2
A Standard Visitor visa https://www.gov.uk/standard-visitor-visa is the appropriate visa for this scenario. You will need to apply separately for each of you, cross-referencing your family membersβ applications to your own as the main applicant.
The PLAB exam is a permitted activity for visitors https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-v-visitor-rules
Work-related training
22 Overseas graduates from medical, dental or nursing schools may:
(b) take the following test/examination in the UK: (i) the Professional and Linguistic Assessment Board (PLAB) test, where the visitor can provide written confirmation of this from the General Medical Council