How to aggregate tags on child nodes?


Let’s say I have a Recipe model, and it is a parent of the Review
model. Reviews are entered by users of the system, with one or more
tags using the acts_as_taggable_on_steroids plugin (for instance).
What I’m interested in is how to show all tags for a particular
Recipe - meaning all tags applied to every child Review of that

For an example of what I’m talking about, look at this page from this
fine website:

They are showing the most popular tags from all Reviews on a
particular Beer. I’m trying to figure out how to do that.

But I have no idea where to start, since the tagging is such a wily
concept to me. Any suggestions?