PWK LCL CTLR EXPERIENCED OPERROR WHEN APPROVING TAXI RWY CROSSING WITH ACFT ON TKOF ROLL.

Narrative

WHILE I JUST BEGAN WORKING LCL CTL AND CIC IN THE TWR; I HAD JUST RECEIVED A RELIEF BRIEFING THAT INCLUDED THE INFO THAT A CESSNA ACFT WAS CLRED FOR TKOF ON RWY 30 AT PWK ARPT. MY ATTN WAS TO AN ACFT CIRCLING TO LAND; THAT I WAS WATCHING ON THE D-BRITE TO ENSURE THAT IT DID NOT FLY INTO O'HARE TFC. THE GND CTLR ASKED ME FOR PERMISSION TO CROSS RWY 30 AT TXWY Y AT THE DEP END OF THE RWY 30. I GAVE HIM PERMISSION TO CROSS RWY 30; WITHOUT SCANNING FOR THE DEPARTING CESSNA. HOW IT WAS DISCOVERED: AFTER I WATCHED THE CESSNA CROSS THE RWY; THE DEPARTING CESSNA CAME INTO VIEW; AND I REALIZED THAT HE WAS DEPARTING AS THE CESSNA WAS XING RWY 30 POSSIBLY. I HAVE NO WAY OF KNOWING IF HE BEGAN HIS TKOF ROLL BEFORE OR AFTER THE CESSNA WAS CROSSED OR BEGAN XING; HE WAS ALREADY ACROSS WHEN I SAW HIM OVERFLY THE DEP END OF THE RWY. I THEN REALIZED THE POSSIBILITIES OF A CONFLICT. THE CESSNA WAS ACROSS THE END OF THE RWY WHEN THE OVERFLYING CESSNA REACHED THE TXWY. SINCE I WAS NOT THE CTLR THAT CLRED HIM FOR TKOF; I HAD FORGOTTEN HE WAS CLRED; WHEN I ISSUED THE XING TO THE GND CTLR. CORRECTIVE ACTIONS: SCAN THEN APPROVE A XING LCL CTLR 1/GND CTLR SCAN BEFORE ASKING. I HAD BEEN ON POS FOR 10 SECONDS OR LESS WHEN THE ACFT XING OCCURRED.