I respectfully disagree. These things may be good motivators for high schoolers, but the OP wants to focus on the younger kids. Fun and games are what keep the younger kids on board--there's nothing that says the middle schoolers have to do the same kind of workout that the HS kids are doing that day.
Moreover, most middle school kids will benefit from a real mix of activity. Break up (most of) their sessions: intersperse different kinds of calisthenics among their bouts of running. Long steady running is deadly for keeping most of these younger children in the program.
One thing that the younger folks really love are relays. These don't have to be formal relays with a baton, on the track; they can be in a park, on a cross-country course, or whatever. Use some imagination. The youngsters thrive on having some element of competition in many (not necessarily all, though) of their practices.
And OP, since you're going to be the T&f coach as well:
1. KIDS LOVE TO COMPETE IN AND WIN (team wins, that is) MEETS. Strive to schedule dual, tri, or quad meets against appropriate competition. You're not primarily developing college/open athletes; you (and certainly the students) are looking to win NOW. (Big meets may not be realistic "win" opportunities for a fledgling program, though you'll want to emphasize those more as your team grows and improves.)
2. FIELD EVENTS AND HURDLES (aka the "technique" events) WIN TRACK AND FIELD MEETS. Your default for a kid who comes out for t&f is that s/he is a field-eventer and/or hurdler unless and until proven otherwise. (Doesn't mean s/he can't do flat running, too.)
A great thing to do with newcomers, especially middle schoolers, is to train them for a pentathlon, teaching them shot, high jump, long jump, and hurdles (all of which may have to be modified some in deference to the kids' size and strength). Don't worry, they'll get plenty of running, too.
3. Make getting a letter a meaningful accomplishment and not just a participation award, BUT be inventive in terms of what can count toward a letter: grades? getting other kids to join the team (and *stay* on it through the season)? perfect attendance at practice? You'll have a better sense of what works in your situation than I will; the key is to be recognizing and rewarding the things that you think will help your program.