> I really wonder if the actual vs imagined crime rate against children has changed since I was a kid in the 60's.
I'm not sure if the data exists for that period, but violent crimes against children in the US are declining sharply. Assault is down 33% between 2003 and 2011, rape down 43%. Matches up with the broader collapse in violent crime in the US.
Now you asked about the 60s, and for murder as a whole (other violent crime follows the same trend), the 30s and 80s were peaks, and the 60s and right now are valleys; if we assume violence against children tracks violence as a whole, it's probable that the the crime rate against children right now is roughly the same now as it was in the 60s (ie, tied for the lowest level since the great depression).
Conversely, I think it's pretty clear that the imagined crime rate (for everything, including children) is basically at an all time high. Polls routinely show that overwhelming majorities of people believe crime is getting worse, even as we settle in for another decade of collapsing crime rates.
So in short, while I'm not aware of any research that answers your question exactly, the answer is very clearly "yes".