One of the reasons I don't think existential intelligence should be ignored, whether or not people think the topic is suitable for children, is that children who have the tendency to think about these kinds of topics will do so anyway.
I wonder if that isn't related to some of the depression that shows up in teenagers. I remember when I was just starting to hit puberty and was being flooded with hormones and had a tendency to feel down about myself anyway, I was particularly stuck on the idea of human existence and what the point of it all was. I had no idea how to process that kind of thought, no idea that there were relevant philosophical debates that could have helped me.
In some ways it was a good thing, because it helped me to think up my own take on existence and I'm happy with it. But, I wonder how many kids with this form of intelligence feel so isolated and confused that they eventually harm themselves? It's a terrifying question for a teenager, what if there is no point to anything? And if the teenager is not given the tools to really explore the question they might give up all together.