Is it possible to enable polling for child resources such as /incidents/{incidentId}/comments?

Polling is not enabled for a generic incidentId. You can only enable child resources for a particular parent Id. For example, if you want comments on a particular incident, you can create a poller path on /incidents/123/comments

You can configure polling on hardcoded specific incident Ids, and the instance will poll on the incidentId child objects as well. Few providers use this approach on the Webinar elements (GotoMeeting OR one of the other ones) where polling is configured on certain webinars to track the number of users signing up and other related activities.