I think the most important thing, when it comes to expressing caution, is to ask yourself why you feel fear and to communicate that. There's so many problems in parent/child relationships, and relationships in general, that are the result of people closing off and not explaining themselves. You can be scared or worried about your child doing something and say so. You can say ''I'm really worried about whether that's a good option for you''. Even if you do mess up and treat them in a stern way, you can still say ''I didn't mean to be controlling, I was scared and feeling insecure, can you explain more about it?''. And it's important to do this at any age of the child's life, that's how you teach respect, honesty and trust to children, and how you establish all these in a romantic relationship as well.