Teaching MA to kids isn't necessarily "lining them up and treating them as you would older or adult students". I've seen some MA-based programs for kids that were clearly mostly about developing the basic motor skills. They had simple tumbling, some very simple kata (balance and limb control), and the like.There are very good reasons we don't teach such young children and that's why he's experiencing something that isn't good for anyone, students or instructors.
JGW posted a very good video of what children should be doing, and it's not lining them up and treating them as you would older or adult students. There should be no fretting about 'stances', or even techniques with such young ones. As JGW also pointed out though, children can easily do more than one thing when they learn. In fact they are the best learners you can get.
Teaching adults as you would children isn't a recipe for success either I've found.