why and how could we send our kid harms way?
I've written a couple iterations of this post and kicked it down several notches from my initial knee-jerk reaction in the spirit of the holiday.
The bottom line is, as usna1985 and others have pointed out, is that it's
not your choice.
Anyway, to address the actual title of your post. I have a more pragmatic answer in me too, but I'll go with the more idealistic one first.
The military
needs smart kids, both as leaders and as followers. Aside from the technical side of things, officers are supposed to be able to think critically and be responsible for large amounts of personnel and material.
A Marine rifle platoon commander (first job out of USNA, probably age 22-23) is responsible for the lives of 45+ Marines, not to mention the weighty decisions he has to make regarding interaction with the local populace or other military actions which can have strategic consequences. Read up on stuff like the strategic corporal, counterinsurgency, the three-block war, and how the military fits into foreign policy decisions to get an idea of how a military officer's job can fit into the bigger picture and have a huge impact.
I don't know about you, but I certainly don't want dumb kids doing these jobs.
And if he (or you) truly decide that the military doesn't need your smart kid, that's fine. I'm sure my class had that too. Don't worry, just as with my class, kids who will go on attend grad school at Oxford, Cambridge, MIT, and Harvard (and in some cases, go on to be major players in whatever sector they choose to in the end, whether it's public or private) will decide that the Navy/Marine Corps is worth their time.