Dessert party will work with you in regards to the allergies, but I don't know how much variety you will end up with in a multiple-allergic kid... It might boil down to Enjoy Life cookies & fruit.
Are you familiar w/BabyCakes at Downtown Disney? Might have some baked goods your DS can have.
How important is an optimal view for you? For me, lack of waiting outweighs "perfect view," so we usually watch the fireworks in Fantasyland, walking up about 5 minutes before show time. For the parade, we always aim to catch the second parade when it reaches Frontierland - about half an hour after published parade time.
You could also pull FP+ for either/both events, for good views & minimized waiting. HTH!