You cannot embed report bodies inside report bodies. You should not need to do that, either. The general principle is that your 'main' body is normally the table with the most records to display. In this case, this would be the KITS table. You would then link other tables to this body, such as kit header info and stock.
Use multiple report bodies only when you want to combine information from completely different tables on the same report, or you want to mix reporting styles, i.e., have a form that includes one or more report sections in it.