A diet that is unlikely to be harmful to a consenting adult may not be so harmless in a 3 year old.

Definitely agree with this, I wouldn't put a child on an extreme starch restrictive or carbohydrate restrictive diet, was thinking more along the lines of the more low risk options like gluten-free/dairy-free, elimination diet, elemental diet etc. There are quite a few studies that suggest that dietary interventions (mostly elemental liquid diets) could be effective for crohn's -

'Diet in the management of Crohn's disease' - http://www.ncbi.nlm.nih.gov/pubmed/6526690
21 of 33 patients remained in remission on diet alone, the most important foods provoking symptoms were wheat and dairy products.

'Treatment of active Crohn's disease in children using partial enteral nutrition with liquid formula: a randomised controlled trial' - http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1856067/
42% remission rate on liquid formula diet.

'Chronic intermittent elemental diet improves growth failure in children with Crohn's disease' - http://www.ncbi.nlm.nih.gov/pubmed/3123302

'Remission induced by an elemental diet in small bowel Crohn's disease' - http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1778272/

'Improved Growth and Disease Activity After Intermittent Administration of a Defined Formula Diet in Children With Crohn's Disease' - http://www.ncbi.nlm.nih.gov/pubmed/1494204

'Mucosal healing and a fall in mucosal pro-inflammatory cytokine mRNA induced by a specific oral polymeric diet in paediatric Crohn’s disease' - http://www.ncbi.nlm.nih.gov/pubmed/10735920
After 8 weeks 79% of children were in complete clinical remission.

'Polymeric nutrition as the primary therapy in children with small bowel Crohn's disease' - http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2036.1994.tb00338.x/abstract

'Improvement of abnormal lactulose/rhamnose permeability in active Crohn's disease of the small bowel by an elemental diet' - http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1433221/

'Controlled trial comparing an elemental diet with prednisolone in the treatment of active Crohn's disease' - http://www.ncbi.nlm.nih.gov/pubmed/2179093
The present study strongly suggests that elemental diet is superior to steroids for treating active Crohn's disease.

Also from a pragmatic standpoint, I'm divorced and my kids are with their father 3 days a week. He has a hard enough time keeping up with doctor-prescribed medication regimens for things like ear infections. He would never consent to a special diet based on my wishes; and even if he did agree to try it he would not be capable of carrying it out.

Thanks for explaining, can definitely see how that would make it virtually impossible.

As for the pubmed link - case reports are not evidence, as seekonk explained.

How is a documented and published case report not a form of evidence? I understand that no strong conclusions can be drawn from anecdotal evidence and case reports, that they are subject to many potential sources of bias and error and are inferior to double blind placebo controlled trials, but I don't see how they can be completely disregarded as evidence. It's bit like saying that a chihuahua isn't a dog - it may look more like a rat and be useless at hunting and chasing burglars, but it is still a dog.