AN ATP RATED CPR PLT FLYING A G1159A NEAR HPN DSNDS BELOW HIS ASSIGNED ALT.

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ON RADAR VECTOR FOR SPACING FOR A VISUAL APCH INTO WHITE PLAINS; NY; APCH CTL CLRED US TO DSND FROM 3000 FT MSL DOWN TO 2000 FT MSL. THE FO WAS FLYING THE ACFT. I SET 2000 FT IN THE ALT PRESELECT AND THEN PROCEEDED TO ASSIST A PAX IN OPENING THE JUMP SEAT SO THAT THEY COULD WATCH THE APCH AND LNDG. THE SEAT WAS STUCK AND DIVERTED MY ATTN FOR ABOUT 60 SECONDS. TOO MUCH TIME TO BE AWAY FROM MONITORING THE FLT PATH. I DID NOT REALIZE IT BUT THE FO WAS BUSY WATCHING ME WORK ON THE JUMP SEAT ALSO; SO NOBODY WAS MONITORING THE FLT PATH. THE JUMP SEAT BECAME UNSTUCK AND I NOTICED WE WERE STILL DSNDING AND BELOW 2000 FT MSL. AT ABOUT THE SAME TIME THE CTLR CAME OVER THE RADIO WITH A LOW ALT ALERT. WE CORRECTED OUR ALT AND THE REST OF THE FLT WAS UNEVENTFUL. LESSONS LEARNED: 1) STERILE COCKPIT BELOW 10000 FT IS COMPANY POLICY. I SHOULD NOT HAVE BEEN ADDRESSING A STUCK JUMP SEAT DURING THIS PHASE OF FLT. 2) WE WERE BELOW 3000 FT AND THEREFORE INSIDE THE SAFETY WINDOW. I SHOULD HAVE BETTER MONITORED THE ACFT FLT PATH AT THIS LOW ALT. 3) I RELIED TOO HEAVILY ON A HIGHLY EXPERIENCED FO (OVER 17000 HRS) TO MONITOR THE FLYING; AND I NEVER CLRLY STATED THAT I WAS GOING TO BE BUSY DOING NON-ESSENTIAL TASKS AND THAT HE WAS NOT GOING TO HAVE ME MONITORING HIM. THIS WAS A CLASSIC CASE OF 2 EXPERIENCED PLTS IN A PERFECTLY GOOD AIRPLANE DURING VFR CONDITIONS; ALLOWING A MINOR DISTR TO DIVERT OUR ATTN AWAY FROM FLYING THE ACFT AND MONITORING THE FLT PATH.