Hi Randy,
I'm assuming you have the song (songs) on the SD card which came with your DP-03SD, and you have not exceeded it's storage capacity - so all of your songs are on the card. The Mixes and Master should wind up there also - if there's enough storage left. You'll have to go through the Mix/Mastering process to see if there's room for everything. The display will notify you if there isn't, then you will need to take other steps.
All parts of your song remain intact on the SD card (individual tracks, all mixes and Master).
Whatever you do, do not format or re-format your SD card - this will erase everything.
You could save that SD card until your granddaughter's additional tracks are recorded; you'd have to keep that in a very safe place for the next 7 years. In addition to that, you can EXPORT the individual tracks to a designated folder on your computer for long term storage. When you do this, take special care to EXPORT "TRACK # _ " (one track at a time). Right next to this operation in the menu is EXPORT "MASTER" , you can do that after you have exported your individual tracks. I would make a duplicate copy of the final folder and place that in a another area (accessed via different file path).
When you're ready, just IMPORT the individual tracks back to your DP-03SD (or whatever DAW you may be using at the time) for remixing and adding new tracks .