We have close to 100 middle schoolers in our program, which is semi-integrated with our high school program. The integration alone helps a LOT. They get to see the faster/older guys training and racing on occasion. The numbers help too of course.
I like to think about middle school being "training to train." By the time they get to the high school level, they should be used to warming up for a workout, cooling down after, what going out for an easy run entails, what interval workouts entails, etc. Details don't matter. Stay away from hammering workouts. Long intervals (repeats anything more than 3-4min) are not great either, just from an attention-span perspective. Keep it simple...10x200m fast, 200m jog is an easy one. Hill repeats are good too: run up, walk down. Pacing for workouts is too much of a mess. Just use effort levels: "Okay today we're gonna do ten 200s at 80% speed with a 200 jog really easy!" You can talk up good pacing a little more on race day, but workout splits, recovery times, all that stuff can wait 'til high school. Remember, middle schoolers have an attention span of 30 seconds AT BEST! As soon as you start going on about workout groups or splits or whatever...you'll lose them.
As for run distance, pace, mileage, all that...not important. Ideally I'd like to get the kids working up to doing five mile runs at least a couple of times during the season, but I don't sweat the details. Plenty of time for that in high school. Just getting them out the door to run for 20-25min and not having them stop to screw around on the playground for most of it is good enough for me!
We do practice 5 days a week, plus Saturday which is optional. Usually 10-20 kids show up Saturday. Usually one game-day a week: soccer, frisbee, ships across the ocean, whatever. This helps keep things fun.
A few kids run in the summer, not many. Not hardcore training or anything...that can come later in high school. Just a few days a week of 2-4 miles. I don't know when the best time to "specialize" as a runner is (i.e. when to train year-round), but my best guess is that it's around age 15-16.
Having goals for kids to work for is nice. "Sub-6 mile club" t-shirts, keeping 7th grade / 8th grade records or top-10 lists, that kind of thing. You already know the most basic point: keep it fun! But try to instill the idea that WINNING is fun too. Take care of the above things, and with the right coaching attitude (focused on the kids, focused on fun, focused on the process) you'll start winning. It takes the right kind of coach...have to be good with kids. The maturity level of 6th-8th graders is very different than 9-12.
Final tip, try to identify talent early. We race a track mile even during the cross country season for a benchmark (good for sub-6 mile club opportunities!). We like to keep tabs on the fast kids...get to know their parents a little better, get an idea of what their other activities are. Most of our top high school runners were also our top middle schoolers. Obviously you don't want to play favorites though. kids pick up on that real quick.