N.K.
At that age they know what the words yes and no means. However, most of the time they cannot fully understand the question but answer yes or no anyways. They cannot yet say "I don't know" or "Sorry but I don't understand" :-) and feel the need to give an answer when asked a question. So they just say yes or no (it will be a yes or a no mostly depending on their personality, the tone of the question, their mood etc).