One thing I would point out is that military kids are in a far different scenario than the OP's cousins kids. Military kids will grow up moving from base to base, but the schools around bases are fundamentally different. Those are schools where teachers are used to student turnover, and the students are as well. OP talked about kids that were in a more nomadic situation, where they could only form brief <1mo friendships. The local culture of these communities is also used to and adapted somewhat to this.

That sounds like childhood trauma to me, the closest analog might not be military children but the foster care system. Not a 100% analog, since presumably the OP's cousins kids didn't have the pre-existing trauma that entry into that system necessitates, but that is the closest conventional analog I can think of. Moving around constantly into communities that aren't necessarily set up to deal with that is rough.