How to prepare parents for B2 VISA interview?

There are fortunately lots of people who have written careful advice on the matter online. I'll link to a few and then summarise. Exmaples:

• Visitors Visa Interview Tips
• Visa Interview Questions for B2 Visa
• Visitor Visa Interview Preparation

What it comes down to is being prepared. Really prepared. This comes under a few categories:

• Documents - bring copies of all relevant documents, in case they are needed to refer to. You'll look organised. Make sure you know what's in them, what they mean, and can find them easily in your collection of paperwork. Make sure NOTHING is missing - missing paperwork is one of the main causes of visa refusal. You should also know exactly where each document is - index them if you find that easier. You don't want them to be shuffling papers around while the irritated official waits for them to find that old piece of information.

• Questions - this is going to be the hardest part for them. They could be asked anything (relevant to the application) but you can look at common questions (second link above) and practice them. Make sure you have the documents for the questions - eg if there's a question like 'what funds will you use to support yourself' - they should be able to produce bank account statements from their paperwork.

• Intepreter - note that it is possible to ask in advance for an intepreter, if you feel that would help them if it's easier to use a family language.

• Extra - three rules. Do not argue. Do not ask unnecessary questions (that can lead to problems), and be polite, no matter what. And if they don't understand or misheard a question, it's entirely appropriate to ask the interviewer to please repeat it more slowly/clearly.

Good luck!