Currently, you don't serve files etc unless you manually pin them yourself.
You share:
1. Files you Pinned (think of as torrent seeding)
2. Files you have in your IPFS cache
a. Cache files are added to when you request IPFS content
b. The garbage collection triggers and removes non-pinned content at regular intervals
To answer the GP's question: As long as you don't pin child porn, and you don't look for child porn, there's 0% chance in IPFS-land.
a. Cache files are added to when _something causes a request for_ IPFS content
If content on IPFS (ie a web page?) can reference and load content from other addresses (assumption) then could someone end up in the situation where they are "hosting" (from the cache) something they would not expect to be? (until the garbage collection clears it).
If this seems far fetched, A submission to HN the other day seemed to surprise a few people[1] as it made a http request to a adult website to check if they had an active session (but did not display any content).
Child porn already exists on the web directly. And Tor Hidden Sites. And Freenet. And other places.
The real problem with CP is that fact there's no mens-rea requirement. A script can download it to your browser cache under blank images. It's in your cache, and you have no clue.
In the current situation, you are illegal. With mens-rea, you aren't - there was no willingness to get it, therefore you aren't at fault. Think of this as shoplifter compared to something that fell in your cart you missed. Same idea.
(In all honesty, I hold to Stallman's idea of CP shouldn't be illegal, period. It's a proof of a crime. Snuff videos of people being murdered isn't illegal, although the murdering part very much is. Child abuse is illegal, as well it should be, but proof of child abuse shouldn't.)