Could it be possible that at the time the application was submitted ( a year or two ago), the he was in the age of legal dependency? Seems like a new law reducing dependent age from 22 to 19 took place August 1, 2014. If the application was was asknowledged in July, when dependent was 21, then he turned 22 August 1st 2014 (for example), it would make him 23 in 2015 and it is also within a timeline for paper-based PR applications.
Assuming spouse/child situation (got married because gf was already pregnant or got married and consived a baby right away) also falls within 1 year time line the application was in process and were simply not declared at the time of landing. Which creates another potentially lost case.
Well, it is a super theoretical situation, I am curious what OP has to say
you are completely right sir.