In LALA app there is a feature to create pin why it is not a 4 digit pin and other thing that why they have not enabled it through finger print sensor there should be both option as it is easy to use finger print sensor as it would take very less time
Fingerprint is a good option, either they can select the face recognition feature also, so if someone doesn't have fingerprint sensor mobile so he/she can use face recognition mode.
Fingerprint sensors are now like a basic function in every phone but the face recognition is a new thing ,so Fingerprint Sensor has to their and its a good security feature as well for LALA world App.