The price calc is per product ID not parent+child.
If you dont use stock, you can perhaps group all child in main product ?
I know sometime it's not possible, so it's not the ultimate solution.
Perhaps this can be handel with a calculation rule, i never checked if this is possible with a calc plugin because the calc is per product ID too but you can perhaps get the current cart content in a trigger.
Another solution can be to create a specific customfield plugin but i'm not really know if this solution have better trigger for this.