How to mount XPath query with taxonomies

[asanchez]

I would need to get the following query:

I have the following taxanomy:

I am going to receive the id of the parent nodes (those that are surrounded in the image with a blue box) and I need to obtain an XPath query to obtain the nodes of the child list (the nodes surrounded with a red box)

[jeroen.hoffman]

How about just this one: content/taxonomies/Facet/Facet/reference-date/element(*, hippotaxonomy:category)?
Instead of the path reference-date you can try selecting by @jcr:uuid, e.g. //*[@jcr:uuid = ‘your-uuid’].

HTH
Jeroen