Explain that to him. Talk to him, explain why things are the way they are. And of course, compromise as you would with an adult. Is your opinion really an universal truth or is there some compromise you can reach?

For example, playing with knives is not a good idea, but maybe all the child wants is to be included in cooking? Giving him a spoon might be enough (and much safer).

But every situation is different, it's not easy. :) Don't give up, and good luck!