Babies are innocent, in that they know nothing of evil and don't know the first thing about sin or their need for Jesus. Yes, all are born with sin natures, but babies don't begin to sin until they're old enough to begin to make choices, good or bad. Even then, they're not yet old enough to understand what sin is and what damage it does, or what their eternal fates will be. For that reason, there is not one single baby or toddler who would be capable of understanding the plan of salvation if you attempted to explain it to him, since he lacks the mental development to comprehend such things.
If God rejected them because they lack the knowledge to accept Jesus into their hearts, every baby and young child who died would go to Hell. That means that, since there's no such thing as limbo for babies (to borrow from Dante's Inferno), they would suffer the anguish of burning in the Lake of Fire for all eternity, just because they're too lacking in knowledge to be able to ask Jesus to save them from their sins.
God is a far more merciful God than that! True, those who are old enough to know and who refuse to act on what they know may well expect to meet their eternal fate, but God does not judge an innocent baby (yes, I use the word deliberately, given what I just typed above) by the same standard He uses for people who are old enough to know. Babies and young children are covered until they reach the age of accountability, even if their parents are not covered by the blood of Jesus. For that reason, if they die, they go to Heaven, and will be resurrected when the trumpet blows. If a young child is still alive when that same trumpet blows, calling up all dead Christians and all children who died while young from Pentecost onward, he will be caught up, too, for the same reason. It is not their mothers and/or fathers being saved that covers them in the sight of God, because no child or adult can ride to Heaven on the coattails of his parents. It is their mental state as described above, whether they're capable of understanding or not.
And, no, the concept of the age of accountability does not mean we can lose our salvation! Not in the least.