An important and probably decisive principle of the Bible is not only found in the fact that 1. the age of 20 is overproportionally often mentioned in the Bible (32 times), that 2. most of the kings started their office at or shortly after the Age of Accountability of 20 years, but 3. especially in Num 14:29-33, where God excluded children up to the age of 19 years from the condemnation of their respective tribes:
... in this desert your corpses will fall, and all your counted ones, according to all your number, from twenty years old and above who grumbled against me. [...] But your little children, whom you said would be plunder, I will bring them, and they will know the land that you rejected.
God punished here the people of the lying spies, but intentionally excluded their children.
The parallel passage is found in Deu 1:39 - with the very important addition that those children do not know good or bad:
And your little children, who you thought shall become plunder, and your sons, who do not today know good or bad, shall themselves go there [the Promised Land which can be paralleled to our future Promised Land], and I will give it to them, and they shall take possession of it.
If God excluded in 1446 BC children under the age of 20 from condemnation, then He will do so especially after the First Coming of Jesus Christ. It would be abstruse to assume the contrary.
Furthermore, children are described as 'innocent' in Jer 19:4-5:
... they have filled up this place with the blood of the innocent, and they have built the high places of Baal, to burn their children in the fire ...
1Cor 7:14 saying 'they are holy' and Mat 19:14 stating 'to such belongs the Kingdom of Heaven' make it clear that all children of true believers who will have died until the age of 19, will be part of the New Jerusalem. Meanwhile, children of unbelieving parents who will have died until the age of 19 will form those nations that serve the New Jerusalem.
Isa 60:14-20 ... And the children of those who oppressed you shall come to you bending low ... [...] And you shall call your walls Salvation [...] The sun shall no longer be your light by day ...
(See also Rev 21:24-27, Rev 22:1-2, Hos 2:18, Mic 4:1-5 and Luk 19:17-19 for more details about the surrounding nations, a topic commonly overlooked in eschatology. All unbelievers not found in the Book of Life will be in hell, but their children who will have died until the age of 19 will form the nations outside the New Jerusalem).
Hope that clarifies the question which is still very relevant.