That's a great way to explain the age descrepencies between characters (if some are 15 and others are 10). Aside from just having all the Kids be siblings.An optional story hook might be if the older kid is baby-sitting the younger kid and something happens. Girls mostly did baby-sitting here in the States, but occasionally boys did, too. The book series and show The Baby-sitters Club was popular in the 80s here and made baby-sitting seem pretty cool to get older kids to look after younger kids. That would be a way to have a non-related younger kid in the group.. or even a reason for the Mystery to take place if the kid goes missing.
I can imagine a situation where the Babysitters are at a house and they fall asleep. They wake up at midnight or one o'clock and the parents still haven't arrived. An uncomfortable situation slowly takes on a more horrific feel as the Mystery sets in.