Have you forgotten what Paul said regarding the knowledge of good and evil, and the age of accountability?
I was alive once without the law, [talking about the time before the age of accountability] but when the commandment came, [when he acquired the knowledge of good and evil] sin revived and I died [he died, not physically, but spiritually]. - Romans 7:9
It's what happened in the Garden of Eden, Adam and Eve were alive once, before they partook of the knowledge of good and evil, but when they did, they died to God because they were condemned by that knowledge, whereas before, even though the law was present, they were not condemned by it, because they had not partaken of it.
In the same way, if a child does not know the law (the knowledge of good and evil) then they cannot be condemned by the law.
Now we know that whatever the law says, it says to those who are under the law, that every mouth may be stopped, and all the world may become guilty before God. - Romans 3:19
But he who did not know, yet committed things deserving of stripes, shall be beaten with few. For everyone to whom much is given, from him much will be required; and to whom much has been committed, of him they will ask the more. - Luke 12:48
http://www.biblegateway.com/passage/?search=Luke12:48&version=NKJV