How do you reset my baby girl from Nintendo ds?

That depends on the game. Usually there is a menu option to delete your save file. For a lot of Mario games you can hold L+R+A+B+X+Y on the Nintendo logo screen to delete your save file.

See the instruction manual for the game.

Note that DS save files are saved to the game card, not the console. So if you play the same card in another DS your save file will still be there.