Best Variation Rankings within Parent
Generally it should be enough to only track keywors for one child ASIN within a parent, as this will also provide the rankings of the best-ranked child ASIN within the parent group.
These rankings can be seen in any Product Dashboard by switching to "Product group":
For most sellers, only the Product Group rankings will matter, as it means the product as a whole is being found via the respective keywords.
Product Dashboard found via Details Page
As shown in this video, you can find the dashboard and track keywords via the details button on the right of each product:
It should also be noted that parents aren't actually real products, but merely artificial groupings of variations, so they themselves do not actually have any rankings.