Your Strategy: Resist the urge to start out with a reprimand, and instead have a discussion with your daughter to find out possible reasons why she's not practicing, and how you and she can problem-solve. Help her get a clearer picture of the impact of her actions by asking questions like, "How much practice do you think justifies paying for your lessons?" Together, plan out how she can fit in practice time with her schoolwork, extracurricular activities, and social life. If your daughter is part of that plan, she is more likely to own it. Or the process may help her realize that she doesn't want to put in the commitment necessary and instead wants to choose another interest to pursue.
Your Strategy: If your rule is that allowance money is to be used for movies with friends, then stick to it. Tell him, "I get why you want to go to the movies with your friends. What can you do next time to make sure you have enough money to pay for it?" Missing the movie will likely motivate your son to budget his money differently.
Your Strategy: This is a great opportunity to try to help your child harness her desire to be in the school play as motivation to focus on her schoolwork. You can ask questions like, "How will you balance rehearsals and homework? When would you get your studying done? What if you fall behind on your school work?" Then map out a plan, and have your child write down the ground rules. You might set guidelines such as, "Every day you will come right home right after rehearsal, turn off your phone, and study for X hours." In addition, establish explicit expectations, such as: "If you can get every homework assignment in and score at least an 85 on your tests, then you can try out. If you cannot keep up with those requirements, you will need to drop out of the play."
Your Strategy: First off, tell her how proud you are that she was honest with you. Then ask her why she tried the cigarette -- and really listen to her response. While it's tempting to start lecturing her on the detriments of smoking, there is a reason she did it, and that's what you want to address. Maybe she was just curious, or perhaps she wanted to fit in. When she feels comfortable speaking with you, you can figure out what's behind her behavior and problem-solve together other ways to feel comfortable with her friends that are not hazardous to her health.
Of course you want to emphasize the dangers of smoking, but broach this topic by asking her what she thought about smoking. Then use this as a way to bring up the negatives -- the horrible taste it leaves in your mouth, the lingering smell on your clothes and hair, and the negative long-term health effects. If your daughter knows that she has a safe space to talk, without worrying that you're going to yell at her, she'll be more open to listening to your ideas and advice, and you can get your points across in a more organic way.
Finally, explore what she thinks an appropriate consequence should be, with you ultimately making the decision. It may be that the coughing fit and gross feeling she had afterward is enough.