Thank you! I had a lot of fun writing it, even though it took several weeks to finally finish it up and share it with y'all. :3
That's a good question. I am thinking that, really, it would just be a case of transferring Young Tiki's clothing, and possibly facial features, onto Adult Tiki. Compare the two:
Obviously the main difference is that the former looks much younger than the latter: the difference seems to be akin to that of a young teenager and a twenty-something-year-old. The basic elements - hairstyle, clothing type, tiara, cape - are much the same, or at least similar, and it would only take some aesthetic tweaks to make it work out. That's easily fixed, and characters like Villager, Wii Fit Trainer, Little Mac and Bowser Jr prove that it can be done.
The real issue here is that their body types are so vastly different: Young Tiki resembles a pubescent girl, while Tiki is fully developed and has a mature sense about her. The trick here, I think, would be to change their body type slightly in a way that would not affect the actual character, yet still offer the impression that Young Tiki is indeed younger than the default model. That would mean changing their facial features (larger eyes, softer jawline) and body type (flat chest, less curvaceous hips) to give the impression of youth.
There's also the issue that one is presumably much shorter than the other, but I don't think that's really an issue. Tiki is never portrayed as being particularly tall or imposing: she has a graceful body and long legs, but looking at Young Tiki, you can see that she also has long, athletic legs that would lend themselves well to dashing and kicking, as Tiki's moveset makes good use of. I think you could very easily achieve a sort of compromise here, making Adult Tiki a little smaller and Young Tiki a little taller so they share the same model without looking too stretched or skewed. I can realistically see that being the case, as Adult Tiki's maturity is defined by her womanly physique and serious, calm demeanour, not by being noticeably taller or more intimidating than her younger self.
There's also the fact that, in her moveset, Tiki is designed to appear small and unassuming, in contrast with her big, fearsome Manakete spirit. In that sense, making Tiki slightly smaller than she usually is and having a child alt would actually help define this marked contrast. There's also the fact that characters like Palutena and Zero Suit Samus are of different sizes relative to their counterparts than they are in canon: Samus in her Power Suit is much taller than she is in her Zero Suit, and Palutena is much taller than Pit in
Kid Icarus, yet they are of comparable size to each-other in Smash. Tiki's size being altered slightly to fit both models at once would work just fine, I think.
tl;dr - Young Tiki and Adult Tiki share the same model, and their differences are noted by a change in physical features and voice, rather than by a difference in height.