As a web developer, I would be unhappy trying to deal with that setup with styles alone. Instead, it likely needs actual reorganisation.
For example, it looks like “tax credits” are coupled with “child benefits” which seems to mean all other types of tax credits seem permanently attached (even though that is where I would have to go if I was single, no children, and working poor).
IRS.gov, since it’s also an Engrish site, has different organisation.
There is still a top-level menu (on the main page, it seems to just have chunks of types of taxpayers, be those humans or organisations), a sub-level (drop-line) menu, as well as a side menu and on-page groupings as you have.
On IRS, if I click “Individuals”, types of individuals appear in the drop-line menu. One of them is “parents”.
So, someone who may be looking for child credits can logically be assumed to first select themselves as an individual (not a business etc), and then a “parent”. Child stuff belongs under parents and guardians so this makes sense. A large part of the resulting page is indeed about tax credits and child credits (and those are at the top as it’s quite a long page), and the rest seem to be a jumble of links someone who is a parent might also be interested in. If those lower links vanished tomorrow, it wouldn’t be a huge deal.
Other types of credits are NOT listed on that page, except as the plain link “tax credits” which is on the page and not on any side menus. Clicking “tax credits” takes one to a page dedicated to just credits of all sorts. Therefore, the working poor do not have to wade through things to do with babies. It seems to be good organisation.
Plus points for the breadcrumb on your site, as it definitely helps. One thing your site could maybe do is have the right menu actually be page-dependent. This may introduce the danger of someone expecting the exact same menu on every page, but I think when things are getting this deeply nested, it may be better to rely on the user knowing how to use their Back button and/or the breadcrumbs.
This way, on a main page, the right menu can really be mostly topic headers, which when clicked go to your current style of pages (main headers with sub links underneath). But once someone has gone into the “child credits” area, for instance, other than maybe a single link on the main parents’ page going over to (the higher-up) “overal tax credits” level, the page and the right menu should maybe be exclusively about parents, guardians, and children/education taxes, period.
That’s something you could possibly do with HTML alone, but still, I’d strongly consider a reorganisation of how things are organised, if possible.