The idea is to have children and leaf count on each node and to embed those infos on node and tag items via custom attributes ("data-nb-children", "data-nb-leaf" for ex.) so user can display them with a bit of css if he need it.

Displaying something like that

is done via content: "("attr(data-nb-leaves)")";

I don't know if you want something like that in your module but if so, I've got a PR for you ;)