Figured it out. So here's the deal. In the (parent's) plugin, I originally was using the format href="child"
. href="/child"
didn't work at all, since it made a link to www.my.site/en/child. I should have paid better attention to what that failure looks like. After the first slash, it assumes you are starting at just after the language choice - this means you do have access to an 'absolute' path.
So the correct way of writing this was simply href="/parent/child"
. Sometimes the simple solutions are a bit hard to see. Short breaks can do wonders.