Since 2005 Supreme Court decisions found juvenile executions unconstitutional as "cruel and unusual punishment"[50][51][52] and that mandatory sentences of life without the possibility of parole are unconstitutional for juvenile offenders.
So the US already can't execute or imprison children. So that's not the reason for not signing it.
Thanks for this. For anyone interested, here is a list if US executions of juvenile offenders since '76. I was sure that I had read of recent cases, but I must be getting old, as that list ends in 2003.
http://en.m.wikipedia.org/wiki/List_of_juvenile_offenders_ex...
So the US already can't execute or imprison children. So that's not the reason for not signing it.