Do not attempt to put the child at ease. Do not engage in informal conversation about the child's activities or interests. You really don't care. Stick to the test. If the child is shy or fearful, all the better.
Any time devoted to building rapport is time poorly spent.
However, if you are paid on a per hour contractual basis, disregard all you just read. Take you time. Keep track of the hourly rate. This is used to ensure many billable contact hours. The purpose is to ensure that the child who is low functioning, or who you want to be, does not understand the task.