This is a very poorly-set question. The relationship between the muscular and skeletal system is a very complex one even when you isolate the simple act of brushing teeth. The various types of bones/joints/muscles in the face, jaw, neck, shoulder, wrist, hand and fingers all work together. For the primary school syllabus, the student is only expected to have an understanding of the overview the digestive, respiratory, circulatory, skeletal and muscular systems, with a bit more detail on the digestive, respiratory and circulatory systems.

Nevertheless, if I have to attempt the question as a primary school student, I would simply say:

• Muscles in the muscular system attach themselves to the bones in our skeletal system, and pull on the bones to enable movement (1 mark)
  
• In the act of brushing teeth, Kelly mainly utilizes the muscles in her face, jaw, and hand (1/2 mark)
• The muscles in these body parts pull on the respective bones which enable Kelly to open her mouth, as well as to grip and move the toothbrush (1/2 mark)

* Note: The teeth are not considered bones but are still part of the skeletal system. I suppose a more complete answer would then also address how the structure of the teeth allows them to be brushed easily but I have left that out since it is already complex enough.

Thanks Adwin for the very detailed explanation!