aha I gotcha now, and you're right there should be something in there to get a hierarchicaltaxon's taxa.
Looking at the HierarchicalTaxon type in JustDecompile
it appears there is a virtual property Subtaxa
that appears to return what you want, but I can't figure out how to actually get access to it...
I've sent an email to the Sitefinity devs for more info on this property and if indeed this is even the way to get at it, I will report back as soon as I've got something for you, thanks for the feedback!
EDIT: in the meantime it occurs to me that you could use the taxonManager to grab an instance of the HiearchicalTaxonomy
type instead, using that HiearachicalTaxon.Id. It's the long way around but might let you move forward until I hear back from the team.
hope this is helpful!