I'm not sure what exactly you're looking for while posting, if it's for advice on what to do about their reaction or if you just wish to vent. Either way, I completely understand where you're coming from. At their ages, I don't think it's quite set into their minds just yet that suffering is real and it could happen to them. What is empathy anyway and why does it take some longer than others to understand the concept of it? To me, true empathy is when you can put yourself and your feelings beyond yourself and try to truly and personally imagine what someone else's suffering feels like from their point of view. Sometimes these jarring videos just don't get through to people, maybe what they need to make the change isn't to see animals suffering but to see their own loved ones suffering and put it in to a view point where it could be them and their own one day. For example, a good friend of mine lost her child months after being born, it wasn't until that great loss in her life that she started to put herself in the shoes of the animals and what it must feel like to them when they have their young stolen from them only moments after being born.
I truly believe your heart is in an amazing place and you are doing so much good for the animals, the planet, your health and your generation, being vegan isn't easy and it takes quite a while to build up thick skin to the negative comments from others, the disdain toward going vegan for others themselves and just in general hearing from so many people that they could "never do it." BUT... you know what? you're one of the millions of people, probably even safe to say BILLIONS of people who are taking a stand toward a cruelty free way of life and if you simply lead by example, one day your nephews may follow in your foot steps.