It depends on their age and the specific topic you want to discuss.
Straightforwardness is good if your child is also straightforward. But it can also come off as pushing, and make them hide in themselves and refuse to give any response to the topic. In that case, before dropping a bomb it’s better to wander around the topic, ask the child’s opinions, say words of support, and give related examples that are not about your child. If it’s about them needing to change their behavior, then suggesting something instead of commanding would definitely be better. Children hate commands and patronizing, but they are willing to do things that other kids do, so any examples are good unless it sounds like “you see, Chuck has good grades so he will go to college, and if you won’t study like Chuck, then you will fail”. This kind of approach will only make a kid hate Chuck and studying. If it’s about school and a child’s future, then offer help or explain why it’s important and how good grades or knowledge will help your kid later. It’s not about being the best in class or whatever, it’s about making your life easier in the future. I’d make sure that it’s about my child and not about me and my wants.
If the topic is about family-related things, then vocalized support is what I find necessary. No matter if that’s about divorce, a new sibling, someone’s illness or death, or a new financial situation – the child must feel safe, loved, and not alone. Phrases like “nothing will change” don’t work, mainly because it’s just lies, so explaining the situation and all possible outcomes and effects would be the best. That would make the child understand that adults include them in their lives and make decisions considering the child’s existence and feelings. Also, after giving the kid pieces of information, I’d would ask how they feel or what they think about it. And it’s important to let the child know that “I don’t know what/how to feel” is a valid answer. Sometimes they need time to come up with an answer or to understand how something makes them feel, that’s why giving them time and space might help. Simultaneously ensuring them that you’re there for them at any time.
If it’s about drinking and drugs, I wouldn’t dictate them to stop. Because they won’t. It will only make them more insistent on doing that. And there will be more and more fights with no good outcome for both parties. I’d ask if they need help with anything in their lives. If something bothers them so much that they are trying to escape this reality and hide behind the feeling that alcohol and drugs give. I know that suggesting therapy can be met with strong defense, so explaining the whole concept of it and supporting therapy could help the child understand that therapy isn’t about weakness, being broken, or needing some kind of fixing. It’s about help with a huge range of emotions that most people are dealing with every day. Some of them find minor help in hyper fixation on hobbies, some of them – in parties and drugs, some – in retail therapy, and others – in food. But some find the courage to ask for professional help and go through anything painful with understanding and support. Yes, you’re paying for that support but any temporary help isn’t for free either. Kids can be ashamed of asking for help and parent’s duty is to let them know that there’s nothing to be ashamed of.
Honesty, kindness and support are the keys.