Amendment:
Litle Ones scratching their victims may start the zombification process as well. But here we have no intel on what exactly happens. I remember that Little Ones were described as having scratch-marks on their arms, but I am not certain (anymore). So I could depict something along that line: Little Ones scratch themselves in order to get blood or lymph-fluids under their fingernails; and when they scratch their victims, these get infected as well. But considering the quick spreading of Little Ones in Boulder I am not certain about this. So an alternative would be that Little Ones might establish kind of open woundings or even glands at their fingertips which in combination with scratchings from the fingernails would transfer the zombie-agent into the victim's body. Here some riddles need to be solved, but we lack the information to work with it.
Best wishes!
Liam