Retrieve a single page from a page collection

how can I retrieve a single page from a page collection in one line?

{% set second_page = page.children[1] %}

something like this?

Yes you can do that, it will do that if your array is integer based. However, i don't 'think' collections are. One way to get around this is to use []notation but with the slice syntactic sugar:

{% set second_page = page.children[2:2] %}

http://twig.sensiolabs.org/doc/filters/slice.html

does not work

Try dumping page.children and make sure you actually have children available:

{{ dump(page.children) }}