Medications- the general belief is to have an extra months supply of meds in hand but many state funded insurance companies do not allow for this. Talk to a representative from your child's insurance company and explain that you need a one time allowance for an extra month of medications. Many parents have been successful doing this.
Food supplies- it would not be difficult for a typical person to live off of canned or prepackaged foods for a few days but for the person with PWS this could be an extreme challenge. Have an " emergency menu" and make sure your kids know that in the event of an emergency their diet will change for a short amount of time and be sure they know what that menu will look like.
Activities- do not be left without activities for the possibility of loss of power. Puzzles, word search books, coloring books and hand held games are perfect but be sure that they are new and not part of their typical activity choices.
It is never easy to prepare for an emergency and adding PWS to the mix makes it that much harder. Plan ahead and the experience will be a lot easier.