I've always found the taste of granola bars to be very hit or miss.
The great thing about peanut allergies is that it is one of the top 8 in the USA and every packaged food product MUST be labeled as containing peanuts in bold lettering.
I would recommend going to the grocery store and letting your kid pick out 3-5 varieties of granola bars that sound appetizing and that you have checked to be peanut-free.
Take them home and have a family taste test. If possible, only take granola bars that are peanut-free so that if you are down to one, your peanut-free kid can have it. This also makes the food allergy experience more fun and inclusive for the whole family. You can even get creative and make judging forms for the event with mickey-shaped ribbons for 1st, 2nd, and 3rd place.
From my experience, a peanut allergy is the most easily accommodated at WDW. Even the Mickey Bars are peanut-free!