NetworkServer.SpawnObjects() not working for child objects

I'm working on a multiplayer lobby menu. Each player selects a role from a checklist, and once a player has checked a box, it becomes grayed out (interactable = false) to the rest of the players. So far so good. The checkboxes are Toggle UI elements attached to a Canvas, and exist in the scene (i.e. not spawned by the network) with a custom NetworkBehaviour to handle synchronization.

My problem is that the server isn't spawning the checkboxes when I call NetworkServer.SpawnObjects()....

NetworkServer.SpawnObjects() not working for child objects