Air Traffic Controller reported that the Developmental Controller issued a clearance to cross a runway; then changing the instructions to hold short of the runway. The aircraft read back the hold short instructions but crossed the runway resulting in a runway incursion and ground conflict with another aircraft.

Narrative

I was an OJTI (On-the-job Training Instructor) training a developmental with low training time on LC (Local Control). There were multiple aircraft in the pattern; multiple waiting for departure; and a Military Trainer in the pattern. Both runways were being utilized for landing and departure. Aircraft Y was VFR practice approach RWY XXL. Aircraft X was a full stop RWY XXL. Aircraft X was holding short of RWY XXL at taxiway 1. Aircraft Y was crossing the threshold RWY XXL. The developmental instructed Aircraft X to cross RWY XXL at taxiway 1. As soon as they unkeyed their transmitter; they realized Aircraft Y was landing rollout they keyed up again and instructed Aircraft X to hold short of runway XXL at taxiway 1. Aircraft X read back 'hold short RWY XXL at taxiway 1'; but continued to cross the rwy. Aircraft X crossed in front of Aircraft Y as Aircraft Y was at taxi speed on the runway. I believe the developmental recognized and attempted to fix the error in a timely manner. I did not intervein in order to hear the readback from the pilot. This event took place over a very short period of time preventing myself from issuing any contrary instructions. High traffic volume/complexity contributed to this event.