How can we teach meditation to children?
Children have a hard time grasping the concept of single-mindedness or mindfulness, as you have discovered so far.
They also have a hard time using the breath-counting technique. Maybe that is why you have had little success teaching your Hinayana techniques.
I think we both agree that meditation can be beneficial to children. But it should not be forced upon them.
There are some children who are naturally inclined to excel in meditation. They are, however, few and far between. You can have them visualize something they like, or recite something. You could make it fun to meditate by setting concrete goals that bring them rewards. Furthermore, you should insist on training them to sit in full-lotus. They respond well to recitation methods.
Children are at their best when they can imitate. Teach them by sitting with them. Become their role model.