I think there was a previous thread similar to this one. I have been thinking about this for some time, and as a specialist in the field of technology and music education, and as someone that teaches ukulele in my classes, I'm pretty well versed in what is out there.
One question is platform: what are you going to be developing for? iOS? Android? HTML 5?
I'll just add that there is currently no good HTML 5 tuner for ALL instruments. Lots of guitar stuff, but very little for band/orchestra/other instruments. With the bubble of Chromebooks on the market in education, that is a need.
As for apps on iOS, it also depends if you are talking iPad or iPhone.
Guitar Toolkit is a great source for tuning and chord fingerings for just about any string instrument, including ukulele.
Kala Brand Tuner is a great tuner, for free. Granted, I prefer a headstock tuner as you can still tune in a noisy room of 60 ukuleles (a daily occurrence for me), but if you need a tuner--it is there.
Ukeoke is available, by subscription, to play songs with--on guitar it is called "Four Chord."
iReal Pro is much like the old Band-in-a-Box where you can program (or download) accompaniments and play along with a backup group.
Music Memos is a great app for recording what you are playing and figuring out what you did...adding bass and drums. You can also export to GarageBand.
Notion is a great app for music notation, which now includes the ability to add ukulele chords to songs, even on the iPad. You can use tab or standard notation (or both). Note: DaveY, this might solve your issue.
ChordTunes can make some quick and dirty ukulele charts (not a lead sheet, like Notion, but a fake-book like chart)
Tabs HD can show tablature from their accompanying website.
Futulele can be used by students with physical disabilties to play along with fellow students if they cannot actually play a ukulele.
forScore and unrealBook can be used to display PDFs that you purchase or create of your own song books.
iBooks and Kindle have many of the Hal Leonard ukulele library available for purchase, including the Daily 365
The Hal Leonard Ukulele Method (I believe Lil' Rev is the teacher in that series) is available as an app.
Stave n' Tabs can also be used to create ukulele tabs.
So...in reflection, there are a lot of ukulele resources available.
What could be of assistance is an app that shows all of the possible fingerings for a chord (on one page on an iPad) as well as chords in the function of a key (e.g. Standard chords flowing to the tonic chord, including common secondary dominants and borrowed chords).