Should I report leaders who provided alcohol to underage soldiers?

I would never discourage someone from doing the right thing. If you were to report such a thing, I would ask for a PCS or inter unit transfer. It's terribly hypocritical if things are as you say. Just be aware that there is going to be some severe boo boo face and probably some attempt at reprisal.

Usually, I would say it's just drinking. However, you were held to a certain standard and so the leadership should be held to at least the same standard. I would report it to at least your immediate higher leadership. At a minimum, something can be done to change your situation. If nothing is done to the leadership at that point I think you can take it outside of that group. There is never a reason why soldiers should be held to one standard while leadership is allowed to run roughshod over them at their whim.

When I was a PVT the drinking age was 18, so we didn't have these issues.
Later in life I found it very hard to get excited about the Soldiers drinking as long as they weren't tearing stuff up and enforced the standard only because it was the standard (whether you agree or not, standards must be enforced) and yes my recommendations were for light punishments for 1st offenders in cases where no one was hurt, or no property damaged, and they were just drinking.
So in my opinion turn them in to your 1SG and Commander, they are setting a bad example, being biased as they punished one Soldier and disregarded / enabled others to be in violation and aren't enforcing standards.
That said either be ready for the shit storm that will follow and be prepared to catch heck from your peers, and no doubt some NCOs or request a transfer out of the unit.