I disagree. If you had written "spouse" or "significant other" or really any other adult I'd agree with you, but I don't think you can make that absolute statement when it comes to (minor) children.
Letting the child know about the monitoring software would be the good and trusting thing to do, but "unethical" overstates the case significantly. I can imagine cases where it would reflect poor judgment or be inappropriate to snoop in this way, but I can readily imagine scenarios where it would be unethical NOT to secretly monitor a child's communication.
Part of a parent's job is to keep their children out of trouble until the child is old enough to do that for themselves. (Another part, as you suggest, is teaching the child how to do just that.) Absolute statements about privacy kind of go out the window when you are talking about a relationship and responsibility of that nature.
It is not hard to imagine a child (not every child, but certainly some, especially in the "tween" age range) that has the capacity and desire to hide some communication from his or her parents but lacking the judgment and life-skills to keep themselves safe from harm when doing so.
Suppose your child is engaged in inappropriate or damaging behavior such bullying/being bullied, involved in self-harm/cutting, drug abuse, illegal activities, or things even more sinister. If your child has the desire to hide that activity from you, you could tell your child about the monitoring software but then they'll be careful to find another channel to communicate about this activity. Alternatively you could not tell them about the software and have some additional insight into their world whether they want you to have it or not.
Look, it wouldn't be my first course of action, I wouldn't do it without provocation, and I would be very very careful how I made use of the information, but I wouldn't hesitate to violate my minor child's privacy if I thought it was important to protecting her well being. Monitoring text messages is nothing, I'd read her diary if necessary.