When you were a child did you ever receive a sharp pinch on the inside of your arm from one of your parents during mass? It gets your attention. It sure used to get my attention.
Children are often lost during mass with the constant sitting, kneeling and standing. They get fidgety, antsy, and easily distracted. When it happened to me I got pinched. But a pinch is not an explanation.
Children often look to their parents to explain the mysterious rituals and prayers that take place. Yet, they often don’t get a very good explanation. The explanation I got as a child was, “that’s what we do.” I accepted this answer and left it alone.