Five states now have laws that allow the death penalty for perpetrators of multiple sex crimes agains children. The age limit of the children varies from 11 to 14. The death penalty has been determined to be unconstitutional in the case of mutlple rapes of adult women.
According to this NYT article there does not appear to be a strong consensus on the potential value of the death penalty in these cases. Here are some examples:

Mark Sanford, a Republican, said in a statement that the law would “be an incredibly powerful deterrent to offenders that have already been released.”

But Richard C. Dieter, executive director of the Death Penalty Information Center, a research group that opposes capital punishment, said the new laws were largely symbolic, would impose disproportionate punishment and were probably unconstitutional.

There are more arguments both ways in the article.
As heinous as these crimes are the disproportionality argument seems correct. But what to do? We do not want repeat offendors and I’m not partial to paying to keep them locked up forever though this may be required if there is not something that will help assure a significantly reduced recidivism.
Which there is. A straight forward part of the solution should be castration. I do not mean chemical castration I mean snip off their testicals! This seems reasonably proportional and given the nature of the crimes there is nothing cruel about it and if done regularly it won’t be unusual. Oh yea, no reason to allow these perps to store sperm.

It isn’t 100% perfect and some effects can be overcome with testosterone treatments but as part of a comprehensive program for dealing with repeat sex offenders it seems like a reasonable part of the minimum package.