i tend to agree with Pagwatch, it really doesnt HAVE to be under direction - you just need to be aware of ingredients in different formats.
We took out most of DS5's dairy before we got to the dietician as he was a very sicky baby and just kinda gave up bottles himself by around 10mths - just wouldnt touch them. He has had varying tummy troubles, alternating violent diarrhea and constipation. Some foods you could physically see his tummy contorting with cramps so we took out one food at a time until he seemed to have regular type nappies. The most awkward one was definately soy, as we didnt realise we were substituting one intolerance for another , but its sorted now. His gut is healing nicely and hes finally putting weight on (hes 3y and up until recently only weighed 22lb, hes 26lb now ).
The thing thats just occured to me is that there are thousands of toddlers/children who voluntarily put themselves on awful restricted diets (a slice of toast and 2 smarties a day) and the vast majority manage just fine on it. As adults we worry about what they are or are not eating - obviously getting medical advice is never a bad thing - but because we are aware of what we are feeding them we can compensate where necessary/possible.
For instance: rice milk, you can get with calcium, lots of GF are loaded with calories (its a side effect of using GF ingredients) so dont worry that they are not getting enough calories either. Really scrutinise EVERY label, shopping is horrendous for a while but it gets easier. You may well find that its just easier to cook fresh, and most GFCF foods are twice the price!
Eating out is a PITA - but Mcdonalds is fab, just avoid the bread bit, the fries are fine, but not nuggets/fish fingers/chicken sandwiches. Dont touch KFC with a bargepole, or Greggs/subway for that matter. Burger king is fine as long as you avoid the bread (again the fries are fine).
THere are obvious things to avoid: cake, buns, biscuits, pasties, pies, pastry, ice cream, cheese, bread, butter/marg, milk, yoghurt, fromage frais, noodles, pasta, lasagne, margerine,
The most awkward bits are the bits you wouldnt realise: coatings on chips, gravy/sauces, crisps, burgers, sausages, deli-meats, beans(some), juices(some), soups, margerine,
and things you wouldnt expect to be ok: turkish delight, marshmallow, gummy bears, sugar lollies (not chuppa chups), nesquik strawberry milkshake powder!
But once you've sussed what foods are safe it does get easier. And actually if you look at very safe foods: fresh fruit, vegetables, fresh meat, fresh fish, eggs, rice, smoothies, sorbets, etc its actually healthier foods than most people would (on average) eat! Its actually a very healthy diet