Next summer my family and I (including a 3 year-old and a 5 year-old child) will be traveling from Hungary to the UK by car. I will know the date we want to cross the English channel by March, but I will know the time only on that day (depending on how far we managed to drive from home in the previous days, we might be arriving at Calais from Cologne, Brussels, Ghent, Brugge or someplace between these places. So it could be morning, afternoon or even evening).
Is it feasible that we purchase our tickets only when we arrive at the terminal or do I need to book it days (or weeks) ahead and try to make sure we arrive in time? I'd rather not wait for hours at the terminal with the children.