My twin sons have applied for NASS but, are undecided about attending if accepted.
I have encouraged them to attend if only as a part of the whole college application experience.
Regarding NASS: My daughter has now participated in NASS both as a participant and as a squad leader. In 2007, her NASS participation convinced her that USNA was the only place she wanted to be. At the time, she thought the "mini Sea Trials" day, was the hardest thing she'd ever done, but the teamwork and esprit de corps that built up over the 5 days made it all worth doing. She really enjoyed several of the academic sessions, others not so much. USNA1985 said it best: if you DON'T enjoy NASS USNA is not the right place for you.
If your sons aren't all that interested in attending NASS & applying to USNA, I would urge them not to apply/attend. There are other ways to experience "the whole college application experience" that don't prevent other, more highly motivated candidates from participating.
If they attend I would expect the academy to be supportive and encouraging as well.
From the follow on posts, your comment here seems to refer to USNA's support of current Mids. I can tell you that without doubt, 4 years at USNA is very hard work. However, support is there for
anyone who seeks it. Academically, classes are mostly very small. There are writing and math labs where Mids can seek help. Professors will readily give EI "Extra instruction." There is a Midshipman Group Study Program, where upperclassmen hold organized tutoring sessions. There are informal study sessions within companies.
From a non-academic standpoint, Plebe year is hard. It is meant to be hard. However, there are still ways for even Plebes to relax a bit. The Mids I know have all found ECAs (sports, glee club) that provide some relaxation and down time. The Plebe Sponsor parent program provides another emotional support system. Even if the official sponsor parents aren't a good match for a Plebe, they usually end up being unofficially "adopted" by classmates' sponsors. Roommates and other classmates support each other, too. There are also USNA chaplains on hand.
The current "Dant" or Commandant of Midshipmen, CAPT Klunder, has been incredibly supportive of the Classes of 2012 and 2013 through their Plebe years. I was amazed to see him during a Plebe Summer morning workout, down on the ground, doing pushups alongside a struggling Plebe, calling encouragement all the while.
All that being said, making it through USNA is very hard work, and it's not for everyone. If your sons want it, they'll take charge of their own application process. Follow their lead. Every year, a few people make it all the way through the application process and into Plebe summer only because they thought their parents wanted them to go to there. Even those who really want to earn that appointment will have times of doubt, but those who are there for any reason other than their own desire are probably not going to make it.
). It took a lot of research on his part and a lot of tough decision making to determine that he still wanted AF, no matter how great NASS was!! (And he just declined his Navy appointment yesterday. 
).