IMO, a child who dies young has been blessed in a way. They are gauranteed paradise because they're not responsible for any sins they have comitted. I hope by that you don't think I'm trivialising the pain of those left behind. I guess to someone who doesn't believe in the afterlife a short life would seem a huge tragedy because that's the only life they see but if you believe in eternal existance the human life span in less than the blink of an eye even if you live to 100.
I guess that it does seem unfair that some people get so much and others get so little but Allah is fair so in the afterlife it all evens out. Suffering on Earth expediates our sins so in a way that too is a blessing.

Just my opinion