There are a defined number of Precheck slots available. TSA determines who gets those slots, first, those with a KTN, then if there are still slots leftover they use an algorithm to determine who gets those slots that meets a certain score. This has long been policy that those between 13 and 17 that are travelling with someone with a KTN are eligible to be selected, it is not a for sure thing, it may very well be they changed the algorithm so it's more likely.
The only thing the airlines have to do with it is transmitting the passenger info to TSA to be checked, either by KTN or to be run through the algorithm.