First, Paul said they had to work around child labor laws because of Diane Lane's age. But she was 18. Would child labor laws still count since she's legally an adult?
Perhaps she turned 18 near the end of shooting, therefore they had to work around child labour standards until her birthday? It could also be a thing that if she was 17 when she started, her contract wouldn't grandfather in the fact of her being age of majority in the middle of shooting.
Also, presumably there were some minors in the crowd during the concert scenes so they would all have to be done during sanctioned hours.
This is just off the cuff, I haven't researched California child labour regulations