We walk all the way through the line with them and then let them ride.
We mostly do this to ensure they do not get bullied in line.
What sort of bullying are you concerned about?
My son is 8 and with
some rides at Universal when we have the Express Pass, he just goes on his own. Flight of the Hippogriff and the Woody Woodpecker coaster being the key ones. The lines are *fast* with EP and it just seems ridiculous to walk through the line with him for 2 minutes, meet him at the exit, and do it all again. So we take some well-deserved bench-time and meet interesting people.
But I haven't found myself in a situation where I feel like sending him into a line on his own at WDW. FP is nothing like Uni's EP, so there will almost always be a wait at WDW, and he might get bored and maybe even lonely. Wait, he did do a 25 minute line at
Disneyland, for the Gadget's Coaster there, but almost the entire line was visible from the outside, and I could see how he did. He wasn't rude to anyone, he didn't accidentally bump people, he chatted with people, he did GREAT, and that lets me know that he's really picked up the kind ways to wait in line and pay attention so you don't back up the line, etc. Obviously I'm more concerned about how HE does over what others do.
When people cut in line when we're all in line, we don't do anything but grumble. I keep the specter of the famous fight in line for the teacups from a couple years ago in my head, and remember that NO line cutter is worth lasting injuries, civil lawsuits, or arrests. And I want him to adopt that attitude as well.
So...a 12 and 9 year old in line together? It's really up to the kids, how well they get along, how well they do in lines anyway (do they swing on the ropes/chains/rails? pay attention to the movement of the line?), will they cause problems if someone cuts ahead of them, etc etc?
If I would just be waiting outside for them, I would probably just wait with them. If I had shopping to do, or *wanted* to sit on a bench perhaps with an adult beverage (assuming I'm not at MK of course), etc, then I might just send them off. What I, myself, probably would NOT do would be to go on another ride, because then what if *my* ride had a problem? Just increases the possible complications in my mind, so I would try to keep myself available.