Your questions are based on some unchecked presumptions which, from the Christian Biblical perspective, aren't true.
Premise 1: Childhood Leukemia is bad.
Premise 2: God gave us the ability to know right from wrong.
Premise 3: God allows bad things to happen without stopping them.
Premise 4: God intended for children to get Leukemia when he created the world and mankind.
Premise 5: An unspoken premise coming out of #4 - Which would be that god wants children to get leukemia.
Argument: If God intended for children to get Leukemia then why don't we register it as "good" because it's what God wants.
Conclusion: There is something contradictory about this worldview.
Which of those premises are not correct according to the Bible? #4 and #5 we can outright say are not true according to the Bible. I trust this is an obvious conclusion for anyone based on reading the plain text of just the first few chapters of the book of Genesis - but if you want to dispute that conclusion I can provide the verses to support it.
And #3 is arguable, but much more nuanced in it's situation. But even if we don't say #3 is untrue, there are some things we can conclude based on knowing #4 and #5 are definitely not true.
The Bible tells us God created the world without death and many of the corrupting effects that later came in through Adam's sin.
It also tells us that God will one day eradicate sin and it's effects, removing death and the corrupting effects of sin upon the world and mankind - Which again shows you what God's true will and design is.
Therefore, we can conclude:
1. God didn't create Leukemia as a condition.
2. God never intended for Adam's children to get it.
3. God doesn't want children to get leukemia because if he wanted it to happen he would have made it that way from the start and he wouldn't remove it at the end of this age.
So why do you register children getting leukemia as a bad thing? Because it's a violation of how God designed and intended the world to be. Which is why He didn't create it that way and will one day do away with the corruption we are subject to currently.
Let's start by examining the rejection of (4) and (5).
First, notice how you've worded (4): "God intended for children to get leukemia
when he created the world and mankind," emphasis added. I might even be willing to cede that (4), as written, may be false specifically from a Christian perspective. However, the Problem arises regardless of when leukemia is introduced to the world: I can simply make a new premise (4'): "God intended for children to get leukemia
at some point." And I think this premise is easily understood and easily defended by merely pointing out God's attributes as omnipotent and omniscient: if God does not want some state of affairs to exist in the world, God will not allow that state of affairs. Leukemia is a state of affairs that exists in the world, therefore God intended for it to exist.
Now, we can pre-empt at least one objection to my defense of (4') right away: consider for instance that somebody argues that God doesn't want broken friendships to exist in the world, but God wants free agency to exist in the world more. It would be reasonable to say then that despite God's omnipotence and omniscience that God could not prevent broken friendships because God cares about preserving free agency. And I would agree.
However, this argument doesn't work for leukemia. Leukemia is not a result of human free agency, and there is nothing stopping God from using God's omnipotence and omniscience from preventing leukemia's existence.
If leukemia exists at any point, it is by God's
intention.
So, I am happy to throw away (4) to accommodate Christianity; but my argument still gets off the ground with (4').
Given (4'), and the arguments for (4'), (5) is no longer under threat and needs no further defense as it's really just a corollary of (4'). The only objection to (5) I can imagine is if someone is somehow able to elucidate how an omnipotent, omniscient being wills leukemia into existence (as must be the case, humans certainly didn't do it) but didn't
want to do it: I don't think that's very congruent with being omnipotent and omniscient.
We can argue, "well, sometimes a parent punishes a child because that's their responsibility. The parent didn't
want to do it, it had nothing to do with whether it was beyond their power not to do it." However, I don't think that we would get very far: we would just be entering a microcosm of the PoE in terms of justice and proportionality: is it just to give young children, who have committed no crimes or at least have only committed trivial crimes (much less heinous than, say, genocide), something like leukemia where they suffer immensely and then die before they can even learn anything from the "punishment?" Doesn't our moral compass register
that state of affairs as unjust, too?
Our moral compasses usually tell us that punishments are proportional to crimes, and that it is not just to punish descendants for the crimes of ancestors (just to pre-empt Original Sin-type arguments here).
Lastly, as for the premise that God will eventually clean up the suffering, that's nice; but it doesn't save omnibenevolence. If I slap a child in the face and then say, "whoops, I know that stung for a minute, but here's a candy bar," that's nice of me (by the end), but I still can't claim to be omnibenevolent.