Work-Cover.reopenWorkObject activity is internally calling the Work-reopenWorkObject(which reopens the parent work object) are final rules. So you don't have any other option to ignore the reopen of the parent work object if the child work object is reopened. And i don't find any other API also in order to reopen the child work object alone.
I don't see any plan regarding this requirement in future releases, as the parent case should always be re-opened if any of the child case is re-opened. So i don't think that this is a feasible requirement by you. If you have any specific use case which needs this functionality then you can raise an enhancement request so that our product engineering team can confirm its feasibility of implementation.