I agree.
I just would have though the arrest of a 13 yr old is quite out of the ordinary unless there is quite a reasonable suspicion- whereas I would not be surprised if, for example, the actual threshold of suspicion was lower for the adults. I think using arrest as a tactic (rather than eg just taking in for questioning) for the adults gives a certain level of formality to the interview process - the important differences between questioning and arrest i don’t think are as relevant when talking to a minor as they are less likely to comprehend the importance of the difference.
Also, I am not sure if there are any UK lawyers on here who could opine, but am I right I thinking the minor’s parents would normally be present for any such interview if arrested? But, presumably, if the parents are also under arrest then the role of a designated adult (or whatever the term is for the impartial observer to the interview to ensure fair treatment of said minor) would then be carried out by social services, rather than by one of the adults who were potentially present at the alleged crime?