A nuke, a virus, whatever... The knowledge is nonlonger the bottleneck, it's the tools and materials.
Also, I'm extremely skeptical about AI becoming even close to a child in intelligence.
As for intelligence of a child... It doesn't need to be a child. An aeroplane isn't a baby bird.
If you are defending a system, you must defend against all 100 things. If one thing makes it through, you lose.
I mean… do we have to hold it wrong?
For instance, it is not because you are able to design and release a (biological) virus that you are also able to defend from it (design a vaccine and inoculate the world's population).
If we're lucky, it might be the case for many instances of problems, but there is no a priori guarantee that this holds.