Our Sages add a provision to these important comments: Even though one must say them, he should say them gently, in order that people will listen (Shabbat 34a).
One must be careful about being overly excited religiously. Do not think that yelling and exerting pressure will yield the desired results. On the contrary, speaking gently is the secret to success.
This is therefore the way to speak to one's wife, and all the more so one's child. An adult is not built to withstand constant yelling. How then can a young child deal with such tension?!
One should certainly educate his child, and certainly point out things to him, but everything depends of how things are said. The words of Sages are heard when said gently (Kohelet 9:17).