Today I made the chocolate muffins from here which would not work so well for egg allergy sufferers as you are substituting 3 eggs. However, I have used cornbread with cocoa powder which always achieves excellent results. Then I popped them into Hallowe'en cupcake cases from ASDA and iced them with the Betty Crocker Chocolate Fudge icing which is, surprisingly free from most things! lastly the ASDA graveyard decoration kit was also free from all we were avoiding but would not suit everyone.
There are plenty of Hallowe'en treats available for children with allergies, although most reach for Haribo, but with a bit of imagination and a few cutters, stamps and accessories you can achieve a great deal!
I've just finished decorating, and we're all set!