The word no is a negative. It means, well, no. It expresses disagreement with something someone else said or did. It is a shorthand word that, depending on the context, can substitute for an entire sentence. No can mean anything from a refusal to cooperate (“Will you take out the trash?” “No.”), a negative reply to a query (“Did you remember to put gas in the car?” “No.”), or a plea to someone to stop what they’re doing (a child screaming “No!” as Mom takes them out into the hall during a church service).
The word know implies knowledge. If you know something, it is a piece of knowledge stored in your brain for some period of time.
Remember which one to use this way: the word know is the first half of the word knowledge. The word no is a very short and simple word. If you want to tell your kids no, then keep it short and simple.