Children must pay attention to posture and essentials when learning sit-ups, and can't do actions at will, otherwise it will easily hurt the waist and cervical spine. There are several essentials for sit-ups. First of all, we'd better warm up before we start to avoid nervousness. Then, we lie on our back on the flat, with our legs naturally bent and our hands crossed on our chest or held near our temples. Then relax your neck and shoulders, straighten your head, exert your abdominal muscles, and slowly lift your upper body off the ground. Don't lift it very high, just feel it, and finally slowly return to the starting position.
If children don't understand, you can refer to the following video:
These are the sit-ups I introduced. I suggest that children listen to the coach's command and guidance more in the process of learning Taekwondo, and don't do the movements themselves, which is easy to get hurt.