You're in luck. You can book minors 14 or older on their own PNR without needing to book them as unaccompanied minors as long as they have their own identification. (I'm not sure if their website is smart enough to realize that the 13 year old will be 14 by the day of the flight though)

If not, you can still book what you're looking for, but you'll have to do it over the phone to have the PNRs linked at booking; it cannot be done after booking if you book it yourself