Christine Collins: Fuck you and the horse you rode in on.
Christine Collins: He's not my son.
Capt. J.J. Jones: Mrs. Collins...
Christine Collins: No, I don't know why he's saying that he is, but he's not Walter and there's been a mistake.
Capt. J.J. Jones: I thought we agreed to give him time to adjust.
Christine Collins: He's three inches shorter; I measured him on the chart.
Capt. J.J. Jones: Well, maybe your measurements are off. Look, I'm sure there's a reasonable explanation for all of this.
Christine Collins: He's circumcised and Walter isn't.
Capt. J.J. Jones: Mrs. Collins, your son was missing for five months, for at least part of that time in the company of an unidentified drifter. Who knows what such a disturbed individual might have done. He could have had him circumcised. He could have...
Christine Collins: ...made him shorter?
Detective Lester Ybarra: Dig. You put them in the ground, now you can take them out. You heard me. Dig.
Christine Collins: I used to tell Walter, "Never start a fight... But always finish it." I didn't start this fight... But by God, I'm going to finish it.
Christine Collins: The boy they brought back is not my son.
Answer: When the accomplice is originally questioned he says that killer told him a few 'may have escaped', implying he never knew if they were caught.
Also, it's implied his accomplice a young boy himself was scared of ending up like the boys himself, sadly I think it was done for survival.