Bless her. I had 2 anaphylactic reactions about 12 years ago and I still have nightmares about them, so I sympathise with your DD. It is such a terrifying experience.
Definitely a good idea to speak to your doctor about it and see if you can get some professional help/advice. Does your allergy clinic/paediatric clinic have someone friendly and approchable who could have a chat with your DD to put her mind at rest? Does your DD know where the Piriton and Epipens are? - I know you wouldn't expect a 6-year-old to administer them herself, but perhaps checking that they are in the right place every so often might reassure her a bit?
In the short term, vegetables, fruit, chicken, cheese, milk do provide a reasonably balanced diet - and could you try to get some "Free From" bread, or make your own, to provide fibre, etc?
I think I may have posted this on MN before, but Sainsbury will provide a detailed list of all their foods which are guaranteed to be nut-free. Could you get hold of one of those, and take it shopping with DD and let jer choose foods from the "safe" list? It is very extensive and you don't feel restricted at all when you're using it. (And it's not limited to the most expensive products either!!)