When my child doesn't understand up/down/left/right

Rather than teaching with words alone, experiencing it with the body is the shortcut. Using the body as the reference, like "hands up = up" and "belly button = front," repeat it within play. In daily life too, add spoken cues like "it's up here" and "your right hand." Left and right are especially hard, and it's normal for them to stay fuzzy until after starting school.