According this doc: http://books.zkoss.org/wiki/ZATS%20Essentials/Getting%20Started/First%20Mimic%20Test%20Case
ZATS supports all CSS3-like pseudo-class selectors as described here:
http://books.zkoss.org/wiki/Small_Talks/2011/January/Envisage_ZK_6:_An_Annotation_Based_Composer_For_MVC
and even better here:
http://books.zkoss.org/wiki/ZK_Developer%27s_Reference/MVC/Controller/Wire_Components#Pseudo_Class

But in practice, at least :first-child and :nth-child() does not work at all, they do not select anything.

ZATS query:
desktop.query("#my_window listbox > listitem:first-child > listcell:first-child").as(Listcell.class).getLabel()

ZUL:
<window id="my_window"
<listbox ....>
<listhead>
<listheader label="Name" />
</listhead>
<template name="model" var="user" >
<listitem>
<listcell label="@load(user.name)" />
</listitem>
</template>
</listbox>
</window>

And this same query that does not use pseudo-classes works fine:
desktop.queryAll("#my_window listbox > listitem" ).get(0).getChildren().get(0).as(Listcell.class).getLabel());