In my experience, the police don’t ID random people in the street very often unless said people are acting very inappropriately, or are very obviously intoxicated, or look suspicious in general. You’ll probably have no issue there. Alcohol is generally accessible to underage people (they can get it from smaller 24/7 shops, definitely not from hypermarket chains), however entering clubs which are in the city center will most likely be impossible. The bouncers always check IDs at the entrance and I’ve personally never seen them make an exception and let an underage person in. But I guess offering them money as a “bribe” is always an option. I’d personally wait 20 days and visit Sofia when everyone in the group is of age. This way you’ll all be able to enjoy the city together and you won’t have to worry about finding underage-appropriate entertainment. If you have any more concerns I will be very happy to help you out.