So mankind didn't create the problem and they cannot prevent it from happening either. This is pretty tough one to blame on humanity.
The only question really left to ask is:
Could God could have created genetics where children didn't die of it (or even suffer from it)?
Well, surely God can do literally anything (all part of the deal with being God, really) and since he created man, the only outcome I think that we can take from this is that God chose to make it so that innocent children get leukemia.
I can't see any other logic in this argument... Unless you can provide a different answer (which, of course, I'd be happy to debate)