change password remote lock

Hi,

when on (really) bad behaviour I remote lock our son's iPhone. for a day or so. 😁. How can I change the password that rmeotel ock uses? When I go into our icloud account andsee his phone an press remote lock all I see is....."This Iphone will be locked using its existing passcode..". Thee is no option to hange hte password. How can I do this?