I fail to see how a machine that can hack everything can't also patch everything and make the system unhackable.

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.

If you are attacking a system, you can try 100 things. If one thing works in getting access, you succeed.

If you are defending a system, you must defend against all 100 things. If one thing makes it through, you lose.

Yes - this is why the concern is one for "alignment". Of course in theory it is possible for intelligence to help and do good things - the hard part is making sure that that is what happens.

As for intelligence of a child... It doesn't need to be a child. An aeroplane isn't a baby bird.

"patch everything" - there is no such way in the Universe unless you reduce "everything" complexity to just one electron. The higher the complexity the higher the surface of messing things.

There is no intrinsic physical reason that the difficulty of 'defending' a system is symmetric with the difficulty of 'attacking' it.

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.