The Office of Postdoctoral Affairs (OPA) and the Graduate School confirmed that Postdoctoral Associates can supervise GRAs. They confirmed there are no restrictions to Postdocs hiring GRAs or student employees. The Associate Director for Science also thought this would be a good experience overall.
One thing to note, the OPA mentioned that we should make sure this is allowable on the grant that will pay for the GRA. Check with the CIRES Finance Manager to ensure there are no explicit restrictions about Postdocs supervising GRAs on the grant.