She didn't do a very good job, though, because I don't know what I'm doing.... She's been... subjecting her family to Mixology practice.
Don't bother using the "practice making drinks" action - actually make drinks. Just like cooking, your sim will get better mixing each type each time he/she makes that certain drink. Plus they get wishes to make, for example, an Outstanding Energy Drink, then a wish to make a Perfect Energy Drink. Just keep making drinks, all the different kinds, and that way when you moonlight, your sim will create higher quality drinks (and hopefully will thus not get kicked out for making terrible tasting drinks!). Also, don't forget to make the custom drinks that you learn from other bars and clubs - you get better at those as well.
Plus, making the drinks will help fulfill that skill challenge.
Edited to add: I would like to take this opportunity to say that I really, really like the Mixology skill. I think it's incredibly helpful. I'm not sure which skill challenge it is, but eventually your sim's drinks are so amazing that their moodlets last much longer, and I've noticed that the Crazy drink will alter a sim's traits for as long as that moodlet lasts (gives your sim Party Animal and maybe Insane?). I personally love the Energy drink - I can keep my sims going for an extra five hours before they pass out, plus I can send them to sleep even before the moodlet runs out, something that you can't do with the Buzzed (I believe it's called) moodlet that you get from making espresso/coffee/whatever it is. I'm not sure how much the Skill Gain drink helps, I haven't tested it, but every little bit helps, especially when you're crunched for time.
