This may or may not be "formal" heresy. Even a Catholic child can be badly instructed by very misinformed parents, and may believe some very protestant things.
Those in a position of instructing have an obligation to know what they're talking about. Those in a position of being instructed, much less so. An argument could be made that the minimal obligation of an ordinary Catholic layperson today is explicit faith in the the matters stated in the Creed (Trinity, Incarnation, Redemption, etc.) or reflected in major feasts. (And I think St. Thomas did make that argument.)
I'm not questioning whether or not the child was in formal heresy (likely not imo). Yet because he believed that the Bible was all that matters, then he did not believe what he was supposed to believe in order to be saved. He died a prot and as such died outside of the Church.
The reason there is no salvation outside of the Church, is not only or necessarily due to formal heresy or other acts or crimes, rather, it is because
it is a sin to *not* believe in the Church, which is Christ. This is all that is necessary to die outside of the Church. Dying in a state of unbelief is to die in sin, the sin of unbelief which. So if anything, we can say he died in sin, the sin of unbelief. Whether or not it's possible for a 10 year old to be in formal or material heresy is entirely irrelevant.
In John 16, Our Lord said: [8] "And when He is come, He will convict the world of sin, and of justice, and of judgment. [9]
Of sin: because they believed not in me". No one can believe in Him and at the same time not believe in His Church. As Pope Pius XII said referencing St. Paul, Christ and the Church are one:
"The doctrine of the Mystical Body of Christ, which is the Church, was first taught us by the Redeemer Himself."To further elaborate, Fr. Wathen puts it like this:
"...And you will say; well they believe in Christ, and my answer is, you forget that Christ and the Church are one. If you separate the Church from Christ, you have sought to divide Christ. There is no such thing as Christ without the Church because the Church is His Mystical Body.As St Augustine referred to this mystery; the Church is with Christ, the whole Christ, there is no salvation independently of the Church anymore than there is salvation independently of Christ. You can never separate Christ from the Church because the two are one. And this is of course the great heresy of Protestantism, they dare to separate salvation from Christ because they separate salvation from the Church. And I’m going to ask you, by what power can they be saved?" So going strictly by Byz's description, then all we can say is that he died in the sin of unbelief and on that account, he died outside of the Church where there is no salvation.