a) No need to pre book - pay the driver or (at Barcelona Airport) there'sa kiosk by the bus to buy tickets from.
b) The 2 I was on were only half full. From reading on here it seems like everyone takes them to Cambrils PArk but we didn't experience this!
c) Departing airport was about 20 minutes late. Coming back was about 10 minutes late, Allow time to transfer between terminals (20 minutes)
d) No. Our 3 year old was free. Coming back the driver told me she had to sit on my lap but there was no need as the bus was only half full and he never mentioned it to me again.
The train is an option depending on your flight times. For me it didn't work due to timetable and my arrival time, If you are taking the train get off at Salou and grab a taxi from there - its closer than Cambrils station to Cambrils Park.