hazeyjane
I know it's very difficult, it took time with DN but in the short term the focus was on making the things she liked and would eat more nutritious and in greater quantities. I was doing her childcare 8-8, 3 days a week so I had to be involved.
The most important thing was seeing a doctor who understood gut problems in ASD who worked with an excellent nutritionist. He looked at her food allergies and intolerances, specifically what foods were causing bowel problems - in her case dairy was a considerable issue, tested her vitamin and mineral deficiencies and supplemented those necessary. (Low mineral levels are very common in ASD). We focused on her daily intake of protein, complex carbs, fats and sugars, and battled to get it to normal levels.
So if they like cake - make it from wholegrains or nuts - ground almonds/pistachios that provide nutrients and protein, add banana, apple, carrot, beetroot, pineapple, rather than high quantities of refined sugar. The sugar in complex carbs is metabolised more slowly and provides a more sustained energy release.
As she liked fruit smoothies we added vegetable source protein powder (pea protein isolate, brown rice protein), and highly nutritious superfoods without her noticing.
She quite liked crisps - or rather she liked the crunchiness but not the taste. So I got her onto buckwheat, rice or oat crackers, wholegrain tortilla and vegetable chips, and carrot and celery sticks.
Not only did her malnourishment and nutrional deficiencies correct, but there was also marked reduction in behaviour extremes - hyperactive episodes, mood swings and lethargy.
I would recommend Professor Brostoff and his work wholeheartedly, and others who follow suit.