When you put क्या before a statement, you turn it into a yes or no question. So, 'वे चाय पीते हैं' is 'They drink tea'. By putting a क्या in the beginning, you turn it into 'क्या वे चाय पीते हैं? which is ' Do they drink tea?'.
The other use of क्या is that it functions as the interrogative 'what'. When you use it this way, it must come right before the verb. Eg: वे क्या पीते हैं? is 'What do they drink?'
वे क्या चाय पीते हैं is grammatically wrong. If you want to know what kind of tea they drink, you need to use कौन सी (which). So, वे कौन सी चाय पीते हैं? is 'Which tea do they drink?'.