Make sure they understand that "I'd check the procedure" and "I'd ask my supervisor" are usually reasonable answers to give if they don't know the answer to the question they're being asked.
Common to get questions about the task they're doing (e.g. procedures, records, work instructions), training, protective clothing they're wearing, hygiene rules, HACCP (or HARA, in the case of packaging) if they're doing something for which that is relevant.
If they're doing something that looks "wrong" (wearing jewellery where it is banned, for example) then that can also often be the source of a question or two, but hopefully this is one you'll not have to encounter 
The other key thing to emphasise is that they don't need to panic, it's not an exam/grilling - so many people get flustered by an auditor talking to them and instantly forget everything as a result. IMEX auditors are usually pretty good about this, but you can reassure everyone in advance.