With fingers 2-3-4 tilted at a 45 degree angle. If it hurts for them to have the nail side on the keys, then let them try it with the flesh side - oh, and press the damper pedal.
They love the sound and it should never hurt to play the piano. If it hurts, something isn't being done correctly.
_________________________
Then let us all do what is right, strive with all our might toward the unattainable, develop as fully as we can the gifts God has given us,and never stop learning." ~ Ludwig van Beethoven