At this time, you are not able to view multiple/combined child ASINs' data on an individual Product Dashboard. However, you are able to view a grouping of ASINs (like all the children ASINs under one parent) on the Products Stats page, but the data will not be a combined total. The child ASIN data will be listed by individual ASIN stats on the one page of results.
Use the Smart Search feature on the Product Stats page to search by the parent ASIN. This will then display all the related child ASINs' data.
If you export the data, you can manually combine any values in excel.