Yes I find my diet makes a tremendous difference. I can't eat any sweets that have E-numbers in (highly coloured things) or any fizzy pop apart from lemonade and cola and no crisps apart from plain salted ones. Chocolate is okay thankfully! If I eat high fat food or oily food then it gets worse.
I really sympathise with you and your daughter - eczema can be life changing and I don't think people realise that unless they are personally affected by it in some way. Your daughter must feel so frustrated because she is so itchy and sore but can't vocalise her feelings properly so it comes out in bad behaviour. I used to have terrible tantrums and would scream and cry and my parents just thought I was a very naughty child. Even nowadays, when I am having a flare up, I am moody and snappy.
My Dad used to give me 'tickles' where he would gently tickle and stroke the sore bits (like behind my legs and the inside of my arms), it would really relax me and it meant my hands were away from the areas so they weren't being scratched. Your daughter might like this too - I get my hubby to do it now!
A cream I have found really helped is called Eucerin but it stings on sore skin so only use it once the skin starts improving. It helps to stop the flare ups because it is intensely moisturising. Also, perhaps she would benefit from a children's antihistamine? I used to take phenergan as a child and it makes you drowsy so good for helping an itchy child (and a weary mum!)have a good night's sleep. I still have to take an antihistamine every day now.
Any other questions please feel free to ask - I'll help all I can!