Andy,
I couldn't find a canned report or smartlist in GP, however you should be able to query the SQL tables directly. You'll want to look at: UPR00904 – Payroll Master Dependent ACA
The UPR00904 table stores a record for every dependent associated with each employee.
Health Insurance Dependent Coverage (HealthInsDependentCov) field meaning:
1 = Covered
2 = Not Covered
Dependents are marked as ‘covered’ in this table by default
This information can be updated throughout the year, and is tracked on a monthly basis