Forget making stuff up, show the real stuff.
I suppose you have to take account of family feeling, but I suspect you might find at least one or two families a year prepared to let it happen in order to protect others.
Think about how people reacted to 9/11. The same people who probably sat happily through Independence Day and other apocalyptic movies. Suddenly, they knew those were real people being vaporised, and not just real people, but real people from their own country, not some anonymous foreigners.
I saw an older girl who'd been knocked down, outside a secondary school, when I was about 7 or 8 I think - it had happened moments before, so the ambulance hadn't even arrived. I can picture the whole scene now, girl in the road, people crowding round, the fact that she was wearing nylon tights and they had blood on them, and my Mum, gently shepherding me away, but saying "She just didn't look before she ran out". I'm possibly the only 40 year old who still follows the Green Cross Code to this day.