Yes ingonrance is no excuse, but they already know child porn is illegal so that isn't the issue. The issue is if they did all they could do to RESONABLY say they had an adult.
Just like if you stash a boatload of cocaine in your unsuspecting grandma's house and the feds bust her, but they can prove that it was yours and she had no knowledge of it, then in FACT she will not be prosecuted.
Another example, if you are fugitive and your mother doesn't know and they bust you in her house, she will not get busted as an accomplice unless she purposefully and willfully tried to hide you.
The terms REASONABLE and WILLFULL are major in law.