Hello @FILMon ,
judging by your comment on my forum post you have already read about my game, so I will skip the introductions. 
As @Axel have previously pointed out, Tigrinya seems pretty similar to hiragana. There are some games on the market that provide interactive minigames which aim to teach the two syllabary systems in Japanese (although one could argue that you can learn the whole set in two evenings or less 
). Some of the examples I have come across in the past can be found in my thesis , where I conduct an analysis on a couple educational videogames. There is also a game called Learn Japanese with Tako, which is basically a collection of casual minigames that might be a bit closer to what you are aiming for. I haven’t played it myself though, so I can’t tell if it is good or not.
Gameplay-wise I don’t think I will be much help as the learning element in my game is tied to RPG mechanics, which I feel is something a little different that what you are trying to do. That isn’t to say that you can’t incorporate certain elements, like bonus items or experience system. I can however give you some general tips.
One issue which I am not sure how I would approach in regards to your game concerns display of pronunciation in text form (or lack there of). As you have mentioned in your first post that the target demographic of your game are children, I am not sure if standardized pronunciation symbols are the way to go. You might have to make do with sound effects in hope that players will create associations between the visual element (characters) and audio (voice). Also, certain characters seem (to my untrained eye) almost indistinguishable from one another (ra vs. ri), which might pose some issues on displays with lower resolution. I really like the idea of presenting a small subset of characters in a query rather than facing the player with all 150+(?) options or a full-size keyboard and I think it goes hand in hand with the minimalistic feel of your game, which is indeed very lovely.
When it comes to repetition management, it is best to use the Leitner system, but any custom algorithm that keeps track of player’s success rates on individual characters / phrases should do. The key in all educational games (and apps) is to motivate the player to practice what they have already learned rather than just continue to learn new things. Make sure you incorporate a system that doesn’t let the player progress any further until they have demonstrated knowledge of previously learned characters.
Good luck with your game, I look forward to a playable build!
