The topic of this post is: Do toddlers sin?
The answer is: Yes
I have to agree.
I think many here had somehow explained why. But let me give my share on that.
Consider the light shed by the following scriptures:
Rom. 5:12 Therefore, just as through one man sin entered the world, and death through sin, and thus death spread to all men, because all sinned. 13 For until the law sin was in the world, but sin is not imputed when there is no law.
In verse 12, Paul was telling us the truth and fact of the matter that through one man, that is, Adam, sin entered the world, the world that God created including mankind. And so it goes, death through sin, affecting and infecting the world, including all men.
Paul tells us, with reference to the law of Moses (that given by God to the people He'd chosen, the Israelites, under covenant), that even before the law was given, the law that imputes sin to the transgressor, sin was in the world. The fact spoken in verse 13 is that sin was in the world. What sin? As one have put it, sin is "missing the mark". What mark, but that, of what is good. All things, be it intentional or not, that misses the mark, is what sin is.
So, we get to understand that, before the law, while there was no law given to man, as that of the law of Moses, and as in the manner it was given, sin is in the world. The difference obviously is that, because there was no law then that will impute sin, sin can't be imputed, though certainly God can, but not man, for there is no law yet to base such imputation. But that at all does not mean that no one sinned. Rather, it is the exact opposite, that all sinned, even then.
Getting to my point, we all at one time lived without the law, even the Jews, as Paul was. As babes and toddlers, all men at one time lived without the law. Yet clearly, as pointed out above, amongst the babes and toddlers, as it was amongst the world, before the law was given, sin is ever there. And that even while no law imputes it to them, as most of us do not impute sin to any babe and toddler. But as you (AVZ) have clearly pointed out, imputation of sin does not erase the fact that sin was committed, even by babes and toddlers.
In Rom. 7:9, Paul said this "I was alive once without the law, but when the commandment came, sin revived and I died." When do you think was that, if not, when Paul, being an Israelite, a Jew, was referring to his infancy and childhood? He speaks of sin "reviving" when the commandment came. That gives us the truth of the matter that sin was ever within the man (sin that dwells in the man, in the flesh), even from infancy. From infancy, man is corrupted by sin and is born a slave to it, for man is indeed of flesh. It is just a matter of time for that to be evident.
Now, going back to the law. And for what reason is the law? Paul said to Timothy, that the law is not made for a righteous person, but for the lawless and insubordinate, for the ungodly and for sinners. All here, I think, will agree that at some point, God's law was given to all creatures, one way or another. And that strongly suggest that we are all, by nature, fallen that is, born lawless and ungodly.