That isn’t how supply/demand works. If you are creating a demand, which you are when buying the product, you are incentivizing someone to create a supply.
supply and demand is a price seeking theory. you are misapplying the term to use it this way
can you show a causal link between being vegan and a decrease in child labor? my guess is that there is a general trend of increased child labor globally that tracks closely the rise in veganism.
we made milk before we had money. there is no reason to believe it will ever stop