Yes, it's the issue of the parents. I have two nephews of the "won't eat anything that has even just little bits of veggies in them" type. I agonized about how to "proselytize" without proselytizing, for awhile. We get together for meals about once every several weeks, so my diet is obvious to them. In fact the parents told them they were lucky because if they came to my house they would eat nothing but vegetables!
It wasn't
too long before the older one asked (maybe at about 6yrs?) why I didn't eat meat, to which I replied that I didn't want to kill animals. And at that time I also told him that I did NOT drink milk, either. And when he asked why, I wasn't sure how to put it, so I asked him to give me a little time to answer, but that I would definitely answer. And then later when we weren't around his parents I explained that they needed to take the calf away from the mother, and that they both were killed later. He doubted me because of a movie in school, but I did say the movie was not accurate. A year or two later, the younger one actually said that it was true (what I'd said) because of some book in school! I'll have to see exactly what the book was, because I'm really surprised it would mention it.
In the meantime, I gave them vegan brownies (that they always ate immediately) every time I went to the farmer's market, and recently gave them a vegan birthday cake. So it's been a very hands-off, answer simply when they ask approach. I haven't broached the suffering angle of it, because I'm pretty sure the parents would get upset by my trying to shock them. I guess I'm just putting in their heads that not everybody kills to eat (their dad's a hunter, and he & the younger one bond over learning where to shoot animals so they die quickest).
I just realized - they are both fascinated by my living with a cat, so I can point out that I wouldn't want anyone to eat her, and there's no difference between her and a calf.
As for children's literature/videos, there are a lot of books & such by animal welfare places. An interesting question at this point is how different abolitionist & welfarist outreach to children would be. Isn't most welfarist children's outreach based on "animals are people too" rather than "this is how to minimize suffering"? Here's an interesting story for slightly older children, about a how a "humane" family farmer converted to growing fruit trees.
http://www.animalsong.org/childrens_stories/appleoak.htmlNot a "quick intro" type of thing, of course.