@HopperFan is right that AS2 is a very open queue in HS. The problem with HS is that with the tiering, you're not left with much else that would be useful for a 5th or 6th pre-booked FP. Assuming your whole family does TSMM or SDD for your tier 1, and you snag Star Tours for you all to ride tier 2, what's left other than AS2? Maybe book Fantasmagic, and one adult/child see that show while the other adult/child do the fireworks show?
If you don't want to pair up and leave a child out while the other child and adult go on a ride in EP, you'll need to let them out of your sight or get a same day FP for an adult. Personally, I don't think there's much likelihood of something bad happening in the FP queue at Disney, but your comfort level might be different from mine.
If it makes you feel better, you could send the 9-year-old with your phone. Maybe get one of those clear phone holders on a lanyard. Either the kids call Dad's phone if there's a problem, or you start a video call with them as they enter the line, chat with them through the queue, and have them turn it off when they board the ride vehicle. This way you're able to monitor them without physically going through the queue (which most CMs would not allow, even if you pinky promise not to ride).