There are lots of them, and they're easy to find. They're all on his website. Here are a few.

RMS has made a few comments saying that images of child sexual abuse should not be illegal.

https://stallman.org/archives/2012-jul-oct.html#15_September...

> Rick Falkvinge joins me in demanding an end to the censorship of "child pornography", and points out that if in the US you observe the rape of a child, making a video or photo to use as evidence would subject you to a greater penalty than the rapist.

> The article does not mention that it's common practice for teenagers to exchange nude photos with their lovers, and they all potentially could be imprisoned for this. A substantial fraction of them are actually prosecuted.

https://stallman.org/archives/2003-may-aug.html

> The nominee is quoted as saying that if the choice of a sexual partner were protected by the Constitution, "prostitution, adultery, necrophilia, bestiality, possession of child pornography, and even incest and pedophilia" also would be. He is probably mistaken, legally--but that is unfortunate. All of these acts should be legal as long as no one is coerced. They are illegal only because of prejudice and narrowmindedness.

> Some rules might be called for when these acts directly affect other people's interests. For incest, contraception could be mandatory to avoid risk of inbreeding. For prostitution, a license should be required to ensure prostitutes get regular medical check-ups, and they should have training and support in insisting on use of condoms. This will be an advance in public health, compared with the situation today.

https://stallman.org/archives/2006-mar-jun.html#05%20June%20...

> Dutch pedophiles have formed a political party to campaign for legalization.
[Reference updated on 2018-04-25 because the old link was broken.]

> I am skeptical of the claim that voluntarily pedophilia harms children. The arguments that it causes harm seem to be based on cases which aren't voluntary, which are then stretched by parents who are horrified by the idea that their little baby is maturing.

https://stallman.org/archives/2017-mar-jun.html

> In the US, people convicted for having copies of child pornography tend to get longer prison sentences than those convicted of having sex with children.

> Mere possession of child pornography should not be a crime at all. To prosecute people for possessing something published, no matter what it may be, is a big threat to human rights.

https://stallman.org/archives/2011-may-aug.html

> "Child" pornography is being used as an excuse to threaten all American internet users' privacy.

> The term "child pornography" is dishonest. The censorship of it puts young lovers in direct danger of prosecution.

> Many published works are disgusting, but censorship is more so. In the Internet, enforcement of censorship puts other rights in danger.

> Please support demandprogress.org's campaign against this bill.

There are loads of these. Stallman takes a hardline anti-Censorship view, and he sees any prohibition on possession or distribution of images of child sexual abuse as censorship.

His argument about criminalising children who share images of themselves with other children has a simple fix: stop prosecuting children for doing stupid shit.