No one mentioned this but...
A) your MP3 players need to be tethered to a computer in your grandkids' home, so is it a Mac or a PC? That will make a difference, as software for many of the iPod competitors is Windows only, so be aware if your kids use Macs!
And, B) would your kids want to "rent" or "buy" their music from online sources like iTunes or Rhapsody or the other stores? Renting means that the player they use works with downloaded music from an online store that charges $10-20 every month for unlimited access to its music library. You stop paying, they stop you from listening to the downloaded songs! Buying means that for 79

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