I'll reiterate what I had said on Discord here: when it comes to the possible courses of action mentioned, I lean more towards a concentrated tag gardening effort for child and loli (shota arguably applies here too, but loli is used far more widely in contrast to shota).
This stance is informed by one particular belief - the fact that we will eventually get a female child/male child set of tags (other child too). We 'split' the tags for pubic hair, dark skin, muscular, and furry into gendered forms, just to name a few, and we'll almost certainly continue to see this trend develop, because it allows for more granular searching. As it stands right now, one of the reasons why people would want a broadening of loli is to enable them to search for non-sexual depictions of female children, something which would be resolved by doing the same for child. Some would argue this would lead to tagging issues, but as noted in the OP, child and loli right now already have tagging issues (like, post #5336056 and post #5343358 could be considered sexually suggestive and thus applicable for loli).
This becomes especially evident when searching each favgroup with -loli -child, flagrantly so when searching something like favgroup:14591 -loli -child younger. However it also highlights how we perhaps shouldn't also rely on Pixiv as the be-all-end-all in loli (as always with Pixiv tagging, see アリス); post #5351119 has already been brought up, and proportionally, given how East Asian artstyles can be, it's something you'd definitely err on whether it counts in loli. Without translations and post #5351120 as further context of her being a brat, you could easily chalk it up to a Sakurai Izumi or even Igarashi Futaba situation, where we're dealing with someone who's likely a (pre)teen or JK/just looks youthful/is a young adult/is short and petite, and not an actual loli (which folks would associate with preadolescent female children first and foremost). post #5212560 is another err example, but one that also highlights how taggers are more concerned with more immediately taggable things such as color and clothes than figuring out what age someone might be visually (as the post in question floats around the recommended 20 tag minimum for posts). post #5102301 once more highlights Pixiv's questionability, tagging a petite woman as loli. And these are just a couple examples.
A superset could be somewhat problematic as you'd have to deal with just how broadly loli is used (its leniency being as much a detriment as it is a boon in a tagging context), as I imagine there are those who would prefer that their search for childlike bodies not be polluted by girls they deem too old for the search (plus it isn't helped by artstyles and artist preferences, contrast these two as an example; and then consider how someone less informed would call every example in those images underaged). Plus, I imagine someone searching for loli would not want stuff like post #5216195 polluting the tag either (as Pixiv also tags it as loli, when it rightfully counts more as chibi). Hell, there's even the issue of girls too young for the search, as this case from Comic LO highlights! As it stands, the way we use loli stands in a perfectly fine equilibrium - most weebs use loli to refer to R18+ depictions of young girls of a particular age range (or if not so, just call every flat anime girl a loli, and every breasted anime girl an oppai loli), and allows us to avoid having to use a tag name like sexualized_child-like_body or something similarly ludicrous that would result in any payment processor to raise their eyebrow.
Ultimately, making female child/male child would resolve the issue of not being able to search for non-sexual depictions of either group, and, while requiring an intense tagging effort, would ultimately prove beneficial due to allowing for more granular searching, and allowing us to avoid having to use loli/shota in non-sexual and non-relational contexts for longer. And even if we did go with a superset later on, having female child tagged up already would make the job much easier for a superset.
PPP does also highlight some need-be variables to consider too, such as the preteen to young adult range being questionably covered right now with tags (the late shoujo/shounen to josei range, if we wanted to be weeby about it; age ranges in Japan can be odd though), as well a lack of a non-sexual tag solution for characters with legal age but child-like bodies, i.e. Remilia Scarlet (petite seems awkwardly unfit for this, and finds itself at the cross-section of the whole artstyle issue from above). The solution for these though would not be a superset, but rather more granular tagging. By the point you're already able to search for sexualized depictions of child-like bodies on Danbooru, you're already not constrained by the two-tag search limit, so there's nothing stopping you from stacking six tags using ~ or OR to get a broader search.
