Rocket Network is actually compatible with several comercial programs- including some with free, though limited, demos like Cubasis. Even though it can serve many programs, people who want to work on the same project still need to use the same program.
I looked into Rocket and I think it has potential, but you still have to pay for the service of having your own project on it. Just like any webspace and bandwidth service, I suppose. The more memory/ bandwidth you need the more it costs.
It might be cheaper for your nephews to get inexpensive CD burners (christmas presents?). If they are musicians and use their computers for just about anything it would be well worth the expense. And mail isn't that slow, and probably more reliable than trying to up and down-load HUGE audio files.
Still, if mixing MP3 files seems to meet your needs, they don't take that long to download and I sure would like to hear how it turns out.
Send the click track as a seperate file. To the best of my knowledge there is no way to get it out of the MP3. Just make sure you guys are all using recording software that can import MP3 files or convert them to wave files.
Better yet, don't worry about the click in the MP3, just have them send you ONLY the tracks they add. Then you can mix then as you like on your system with or without the click.
Good luck!
Chris