Pam, I'm not shore on what level you can go moonlight, but you should get a message about it, it is early and it might be at level 2 or 3.
I only played YA mixologist, and whenever he visits the bar that is open and already have an mixologist on duty, I always got the option to moonlight (among other options such as order drink, bar food, give advice etc). Even in SV, not just in BP. Maybe they are teens, or maybe you tried too early, before NPC mixologist even show up? If not, than wait till YA birthday and hope for the best.
Leto, thank you!