I agree. Often a predator can be identified at a very early age (7-12-14). And usually, they can't be rehabed. There was a discussion here once about killing such kids. Don't think I could live with that. But locked away forever doesn't bother me a bit.
I don't think children under 12 are unreformable enough to kill them.
"Locked away forever" creates a number of problems which you should think through.
First, it's very expensive. There are probably 50 000-100 000 eligibles right now, and many of them are wily enough to be out, or in prison for only short stretches. Converting them all to life will cost a fortune.
Second, the non-predator prison population will suffer enormously as a result of your proposal. And you can't put them all in solitary, most men cannot tolerate 30 days of solitary, much less fifty years.
Thirdly, there is the practical problem of escape.
If you could design an isolation facility (a penal colony) which was escape-proof and capable of agriculture, so that our costs would be limited to guarding the wall, I could go for it.
But killing them right away wins hands down, in my book, because it's cheaper, safer, and better for the non-predator inamtes who we will still need to incarcerate.