You can't organize them as child searches, but you can chain them using the Saved Search search condition. (There might be a few bugs if you get complicated, but it should generally work.)

Just so I'm clear on your request, though, you're suggesting that you could right-click on a Saved Search and choose New Child Search, and that search would automatically use the parent as the source while letting you assign additional conditions?

Would you be able to drag a search in or out of another search? It seems like that gets a little weird, because the parent search condition is implicit when it's a child search.

In Jabref, I can make collections in a tree like fassion just as in Zotero. The difference in Jabref is that I can specify for each collection whether it is a manual collection or e search (dynamic) collection. In Jabref there are mor options which makes it a bit complicated.

This should be easy to implement, just allow:
-nested saved searches
-no alphabetic sorting, manually move around searches (drag/drop) also between existing searches/collections)
there are no dependencies necessary between saved searches

This would solve also other comments in the forum related to hierachical tags or keywords, wich are difficult to implement.

A further option would be to have a searched save to include also all other child searches. If I click on 'mammal', it shows this search, but also all child searches: 'dog' and 'cat'

The magic is that if I add papers, they appear automatically in the search w/o having to assign keywords. If 'dog' is not in the title, I can add it as a note and search for title and notes in the search. That way I can adjust papers that do not show the search keyword in the title. I did this extensively in JabRef. It works beautifully.

The kick would be to be able to select a search 'mammal' in the animal tree and to be able to select (ctr-click) another search 'cortex' in the boody part tree. It should show all papers with 'mammal' AND with 'cortex'.

But this gets very complicated. For now just being able to have a nested saved search tree structure w/o alphabetic sorting would be nice.

Thanks

no alphabetic sorting

This seems fairly unrelated. All of your examples are alphabetical within their parents.

there are no dependencies necessary between saved searches

Ah, OK. Nesting searches under other searches without any inherent relation seems a little strange to me, but I understand it for your example. And I suppose it's not that different from allowing searches within unrelated collections, which is planned.

The magic is that if I add papers, they appear automatically in the search w/o having to assign keywords.

I mean, that's just how saved searches work in general. That happens in Zotero too.

The kick would be to be able to select a search 'mammal' in the animal tree and to be able to select (ctr-click) another search 'cortex' in the boody part tree. It should show all papers with 'mammal' AND with 'cortex'.

Other than the hierarchical part, you can of course do this already with tags. We've talked a little in the past about multi-source selection — not impossible that that could happen at some point.

For the record, vintage ticket: https://www.zotero.org/trac/ticket/965
and long-lasting thread:
https://forums.zotero.org/discussion/1027/organizing-saved-searches/
on basically this.
I've always assumed we'd allow nesting of independent saved searches as part of this. I don't really see the downside.