Oh, it is simply that children are more innocent and thereby don't deserve suffering as much as many adults.
Agreed. Over the years of debating theists and the morality of God, and them asking me what would convince me their God exists, I've said that I would be impressed if children never got very sick until they hit, say, 16, the age of consent. If children were able to enjoy a childhood without the lottery of life that hits most adults with fatal diseases, i would think the universe is looking out for these innocent lives. I know some kids can be brats, but are they so bad that they get cancer? And it's not as if the bratty ones are getting cancer. A client of mine had to cope with their 3 year old daughter getting Leukemia, and then die after 2 years of painful treatment. She was a good kid. That really shook me up because I never knew kids could get cancer like that.
I'm sorry, but the excuse that nature functions independently of God is nonsense when that same God created the universe. The God created cancers as part of what it designed. If they want to concede their God is incompetent or a sadist, then OK. It would fit our observations as we compare their claims to it. Theists want to have the cake and eat it too. It just doesn't work as a literal being. At best I think God and most other religious concepts are metaphors, much the way most Eastern Gods are representative of one thing or another. Western religion got into a bad habit of accepting their beliefs literally, as hard concepts. It just doesn't work in the age of reason.