Augustine somewhere wrote of observing two very young infants, one being nursed and the other not, and the look of envy, anger, and furious jealousy on the face of the one not then being nursed, even though his turn was coming, on seeing the other given the breast first.
Was that envy, anger and furious jealousy sin?
I’ve observed enough young children to conclude they are not perfect little angels, to understate things. One of the first words from a one and a half year old is frequently “NO”, shouted when the will is thwarted.
I think there is a difference between when they start to sin and when they must answer for it. All you can do is pray and train them in righteousness, including correcting them using methods they are sure not to like.