Maybe but I have a different perspective. In this area, going to another country is similar to traveling to another state in the U.S.
DS could be in several different countries by bus, train, car, or short airplane ride. So it would not be that big of a deal for him to go to, say Thailand or Cambodia, for a missions trip.
But it would also be kind of redundant since we live in "missions situation" every day.
So my answer is still maybe but the middle school child would have to be mature, grounded in her faith, and need to feel a call to it because a lot of short term missions trips are mostly vacation time with LOTS of teenage drama mixed in.
Friends of ours use to lead STM every year in other countries but they stopped because the drama overshadowed the mission.