My three DDs all have coeliac disease - DD1 was diagnosed at 2.8, my DTs were diagnosed earlier this year at age 4.
As others have said, once the diagnosis is confirmed, there is lots of stuff available on prescription and in all the big supermarkets now. Eating out is an issue, but more and more restaurants are becoming aware - we have a local Italian which does g-f pizza and pasta now and we take our own pizza bases to Pizza Express for them to cook for us. Carluccio's are also now doing g-f pasta.
Once the diagnosis is confirmed, for breakfast if she likes cereal, you can get g-f cereals in the big supermarkets - my girls particularly like Doves Farm chocolate stars and Kallo honey puffed rice. The main thing that is hard to get used to is the difference in the texture of the bread - by and large it's ok toasted, but not great as plain sandwiches, although there's a fantastic new loaf you can get in Tescos called Genius Bread which is, frankly, Genius!
You can buy all sorts of cakes and biscuits, but I've found the Doves range of flours really great for baking - they do a self-raising flour now which I've successfully substituted for standard recipes.
HTH - happy to help with more info as and when you need it...it is hard to come to terms with it, but once you change your DD's diet, you will be amazed by the change in her and how quickly it happens.