FXE Tower Controller in Charge reported they observed an aircraft taxiing across the runway at the same time as a departing aircraft flew over the top of them.

Narrative

I had Clearance Delivery (CD); GC and Controller in Charge (CIC) combined; which was normal procedure for that time of day. A new controller started their shift and I instructed them to take GC from me. I gave the position relief briefing and included in the briefing that Local Control (LC) owned both of the runways and ensured that the memory aid magnets reflected that. I also briefed that Aircraft X (the only moving traffic) was taxiing back to ramp via G; E; but was given instructions to hold short of Rwy 31 at E. After giving the briefing; I went to cut a new ATIS to split the positions and update the CRU-Art to reflect the position opening. While cutting the ATIS I looked out the window and observed the Aircraft X aircraft crossing Rwy31 and saw that Aircraft Y was departing at the same time and overflew Aircraft X in the intersection. I asked the GC if he gave the clearance to cross or if the aircraft taxied on their own and he said that he issued the crossing. The magnets we use for runway control memory aids are not sufficient. We do have LED light strips installed for more situational awareness (per collaboration); but they are not yet in use; as the controllers have not been briefed on the procedure. We need to start the testing period for these lights ASAP to see if these are a more viable alternative to the magnets.