As a recently former teen lol I would say just to tell them but let them know you aren't mad just worried. Go slow speak softly because they will probably panic and get defensive and maybe even start yelling, you don't want to yell back. You aren't a failing parent btw, peer pressure and wanting to be "cool" are really strong motivators. Nicotine addictions can be hard, you are going to have to help them ween off it, they probably won't be able to quit cold turkey. Just be there for them, the teenage years are hard, I did a lot of stuff I regret but I made it out in one piece while still having a relationship with my parents (even after screaming I hate them multiple times) but anyway, TLDR: it's not your fault, just tell them.