Unfortunately, Stuttgart Airport to Heidelberg by train is far from a direct route.

Your parents will have at least two changes of train. They will first need to travel from the airport to Stuttgart Hauptbahnhof (central station), where they will have a platform change, then two more trains via (usually) Mannheim. Overall, the journey time is around 2 hours.

www.bahn.de show the various connections, including platform numbers.

If your parents are uncomfortable with navigating the changes of train, an alternative would be Flixbus, which operates coaches directly from Stuttgart Airport to Heidelberg city centre.

Monday departures are 16:05 arriving at 18:40, and 17:50 arriving at 20:30. Prices are usually around 10 euro or so.

Tickets can be booked at www.flixbus.de.

You can also try www.flixbus.co.uk for English language bookings, but prices are in Sterling.