Repro steps:
- Open SubEmitterIssue from the attached project
- Enter Playmode
- Observe error in the console: "Sub-emitters must be children of the system that spawns them"

Notes:
- The message seems erroneous - there's nothing apparently wrong with the system
- Deleting the remaining particle systems in the scene resolves the issue
- Duplicating the offending system and deleting the original also resolves the issue, so it might be some prefab corruption
- Does not repro in 2017.1
- Project is a stripped down version of "L.V.E - Lava & Volcano Environment" from the asset store
- Seems possibly related to https://fogbugz.unity3d.com/f/cases/1042951/