I enlisted. My parents, old hippies, were NOT pleased or impressed. My extended family ranged from bemused to shriekingly horrified. I wanted to be a Seabee, which is one of the more dangerous jobs they were letting women do.
I think my being a girl was part of their concern but honestly in my family I think they would have been pretty much the same for a boy.
I was peeved at all the people advising me on how to get safe jobs (I think they thought they were helping) or assuming I wanted that, and I was hoping to go to Iraq or Afghanistan.
I didn't make it that far, I washed out with injury and years later I am still dealing with physical effects to say nothing of the huge slam to my hopes/dreams because I had planned the military thing since I was a kid.
Even getting out didn't mend all the things I remember my aunts/uncles/parents/assorted folks saying to me about it and the very few who were supportive (old men, oddly - the WWII generation is the ONLY generation that came through for me, even the kids my age in my family were blah blah blah why would a woman...) are the ones I had the easiest time relating to afterward.
I don't think not supporting the kid accomplishes much. It can leave a ghost that outlasts the dream.