Kids this age need a fast-paced class with small blocks of information. You can barely expect them to stand still, let alone stay in a proper stance for any length of time. If you want them to stay in a stance, make it fun. Tell them front stance for a ten count, then have them count with you. Then switch. This teaches basic counting, a stance, and right versus left. And if all else fails, have them sit when you give them instructions. Sit down, quickly tell them what they are going to do, then have them jump back up (saying "yes sir" or "yes ma'am!") Good luck!