A:Answer The physical copy, you must carry the game cartridge everywhere you want to play it. If your granddaughter loses the cartridge, there is no replacement and the game is lost forever.
For the digital copy, she has to have a Nintendo account set up. (Parents/Guardians can set up a child account that prohibits online purchases) but the game is stored in the cloud digitally. As long as she has access to the account via username and password, she will always have the game.
That's the only difference, a physical copy vs a digital copy. They are both the same game.