Hey, for what it's worth, Elf children being added to the game is something many people have hoped for and I myself have been looking for a decent option for for months 
currently there are two options available that will add dunmer child assets.
The first is an elf children mod for xvision children, its based on the drow race mod and creates horrible, poor quality anime elves that look completely out of place in a lore correct game.
The second is SkyKids, the assets used for this are not terrible, but the textures and models definitely need a lot of work, however it doesn't actually add any non human children to the game and nobody has done an expansion that adds any as of yet.
Then there are two mods that may or may not be getting worked on currently.
The first is yours, which looks like it'd fit in well with the base games children (because you used them as a basis)
the second is an expansion for RS Children that Nanabeats was working on (Linky) however I don't know if she is.
as far as the base children are concerned, I think with the work you've been doing this mod would be an excellent option for anyone that wants a 'full' child overhaul rather than just a fancy overhaul of the human children already in game.
I understand your issues as far as making what you're working on compatible with RS Children, the two are very different styles and for your elf and orc children to fit in with that mod, they would need to change a lot, which is, in my opinion, really not worth it!
I would have one suggestion concerning the mer children though, perhaps look at making their faces a fraction slimmer than they are currently, that would probably help make them look a little more distinct from the human children. 
for voices, have you considered asking over at the Tes Renewal site? I know that they've been doing voice additions for the Skywind mod and you might find someone over there might be more than happy to help (especially if you're in turn happy for them to use what you've made eventually!)
I do think you might find a good way to help motivate yourself might be to release this in stages, release a version that replaces the base npcs in the game as an ALPHA build, helps get some feedback that way, then add the other features as and when they're ready as updates. It'll surely help with the feeling of accomplishment and will surely help with gathering feedback and maybe even finding some other people to help with it!
The less fun prospect might be the creation of patches for the more common mods... (expanded towns and cities, inconsequential npcs, interesting npcs, wyrmstooth and falskaar, I believe are the ones most people are likely to 'expect') but perhaps if anyone was willing to help out in that regard you might be able to get a bit of help from someone else in creating those patches ready for when you're happy to release a beta release!
Anyway, good luck, I hope this mod does come out, it looks great 