Power down the nabi and wait 30 seconds and turn back on and try to log in as mommy mode. If that doesn't work then do a factor reset.
Remember doing a factor reset will wipe your nabi completely to factory new which will loss all your pictures and apps (unless you backed it up).
"Press the Volume + Button first, then press the Power Button and hold them both until the system menu appears on the upper left corner of the screen. 3. Use the Volume – Button to highlight “Wipe Data/Factory Reset,” then press the Power Button to select It. "