8 Yr old girl, just diagnosed with ASD, stealing food

Another really important thing came to mind. 

Our biology requires a set amount of nutrients - vitamins, enzymes and minerals, which are found in foods. I want to specify I'm not referring to a type of 'diet' but the chemical make up of things which the human body can digest. 

If your daughter has only been able to previously get the nutrients contained in milk from milk, then her biology might only recall Milk is the source of the nutrients milk will provide: Protein, enzymes, calcium. Here's the catch: These things do not exist in a "milk alternative". I'll make an analogy: If a person eats a good deal of bananas, they may not ever crave potatoes as these food items contain a similar amount of potassium and the human body won't create a craving for this mineral as our very cells don't wish to overdose. Now, you might be craving potato crisps due to the salt content, as your body might be lacking other minerals to help stay hydrated, then perhaps salted nuts sound good as well. 

Humans who steal are either in a state of desperation or deeply sadistic: drug addicts and corporate overlords or tyrants. I would suggest your daughter is too young to be either, rather, just being human and feeling she has to take matters in to her own hands in order to deal with a natural biological process.

From what I've read, one can substitute blueberries and fish for milk to get the same nutrients. I might suggest even buying lactose-free butter, cheese and whole milk yogurt. And then, I would eat how she eats for a few months or longer, so you can gauge if the substitutions are working. If you still have a craving for milk from whatever food sources you've switched to (I might be careful with cruciferous veg for young humans), then it may be time to invest in a dietician to find out what contains the same nutrients (Not what substitute 'works' on cereal). Soya milk will the protein and enzymes even if it's fortified. But it shouldn't hurt you in the least to go without lactose for any length of time. 

Steeling is giant word for such a little person :(

Is she excluded from what the rest of the family is allowed to have? 

When I found out my son had an egg allergy I stopped eating them as well. It forced me to learn how to bake and cook without them. It was a way of 'suffering' with him (as he loves them). And it's this camaraderie that is often far more compelling than going without something. We all want to feel like a part of something - this is useful in advertising & persuasion. After several years he got used to it and eventually buys his own vegan cakes now.

I used to think I was lactose intolerant and stopped having milk for 15 years until I discovered it was gluten that was the problem. Dairy can be difficult to digest with other IBS issues. Once I stopped eating wheat-gluten and all it's relatives, I could digest dairy easily. These allergies aren't simple! However, once gluten became too painful to process and I was more frightened of accidentally eating it, I didn't miss it in the least & still don't. 

This may sound a big silly but I had ectopic eczema when I was a child. I couldn’t use any regular soaps creams or shampoo. I used to see my sisters washing there hands with a bar of soap or having bubble bath and I’d so wish I could use it on my skin. I used to cut bits of the soap and keep it my room. I would scrape the soap against my teeth just to taste it as I knew I could rub it on my skin.

Could you house hold become Lacoste free? Or as the other user has suggested a fridge lock but I don’t think that would get to the root of the problem. She must feel Terribly left out. 

is you daughter eating the food or taking it and hiding it?