Parent the countdown display to the bus stop cube. Simple.
Timetable set for bus routes?
It might be parent to the wrong object.
Reset the parent to 0 then re parent the bus stop cube to the destnation display again. Also what destnation display are you using btw. Are you using the default object or an freeware object. And if you can, you upload screenshot of bus stop display if it continues not to work.