Coming from the south i.e. Hamburg direction you will be on the E3/A7. Leave at the exit for Eckernförde, B203 and carry on through to the L26 to Kleinwaabs. Turn right there and The Heide Family campsite is well signposted. On the internet "Calculate Route" will give a journey plan.
Please notice, that in the Luchtime between 13h-15h an check-Inn is not possible.