How well do children understand combustion engines? The Christian Bible, 66 books if you’re Protestant and more if you’re Catholic, contains such riches that constant reading doesn’t exhaust it. So to ‘fully understand’ requires some qualification.
In fact the final book, the Revelation, is not comprehensible in any good sense. Some read it in a linear fashion, such that what the middle chapter says is in fact the middle of its message. Others experience it as going over the same material multiple times, each from a different perspective, i.e. it’s written as a group of concentric circles. Note that in Scripture just about every number that appears carries a symbolic meaning with it. There are sects (Jehovah’s Witnesses, notably) which read some of this symbolic language as a literal counting-up of souls to enter ethereal heaven, with all other saved souls relegated to live on “the new earth.” They may even be right, but I’m not ready to adopt their position.
One point to ponder is that in the Gospels Jesus tells his disciples to let the children interrupt their elders, i.e. to cluster around Him. He told them to let the little children come to Him, because any soul not approaching Him in that same manner (you can infer wholesale trust and peace, no worrying about nits and conditions) can’t really approach Him at all.
Hope this reflects well on the question.