How did 10 people (Captain, Maria, the children and Uncle Max) fit into the 5-seater car? The Captain obviously drives while the youngest sits on Maria's lap on the seat beside his...that's 3 out of 10. Did they pile two people on top of someone on the back? Or stuff someone in the trunk?
Unless you consider that Maria could've been holding Marta and Gretl (one arm around/across each. That's 4 out of 10.) Liesl could've had Brigitta, who's 10, on her lap also. Or Louisa takes Brigitta, Marta goes with Liesl. At any rate, nobody said escaping a soon-to-be-fascist country in a 5-seater was a picnic.