The parents of critically ill children benefit was established with the Helping Families in Need Act brought forward by Minister Finley, and has been in effect since March 2013. In order to establish those benefits, essentially what's needed is a medical certificate from a specialist that says that the child is truly in a gravely ill situation.
In order to change to sickness benefits, the provisions are essentially lighter on behalf of the parent. They need to be able to establish from any medical practitioner, say a GP, that they are in fact ill, and they can suspend their benefits and start sickness benefits. Essentially, it's not an onerous requirement on people to switch from one type of benefit to the other. I'm not sure if that gets at the heart of your question.