When we speak to our children we are creating some kind of reality for them, whether we are aware of it or not. The child is born to us at a state that he knows nothing and is looking to us to teach him EVERYTHING about life. Meanwhile for us, speech has became second-nature, habit, something which involves at times no thought what so ever. We must be aware of this and constantly keep our minds awake, to create positivity and not let auto pilot take over. When using negative words with them we CREATE a negative reality for them.
An example from our home- when my daughter uses inappropriate, and aggressive ways to achieve something by speaking to us rudely, we tell her, "this is NOT how we speak in this home. With this tone and method you cannot gain anything." We are now trying to work on answering like this- "In this home, when we want something from the others, we use a polite and calm voice, like daddy is using now you see?" This gives them an EXAMPLE. This is very important and is overlooked many times. We must remind ourselves that they do not know how to react, and if we just tell them what not to do, then how will they improve next time!?
By showing a positive example we gain two things, :)
A) showing them an example they see with their own eyes of what is expected of them
B) telling them what to do therefor instructing them for next time
By telling them negatively we loose two things, :(
A) wasting an opportunity to show them an example of the proper behavior
B) not telling them what to do, so they actually don't know how to act, just what NOT TO DO!
The Torah tells us that the world was created by speech! Hashem created the world with ten statements. We also say everyday in Shararis "Baruch SheAmar Vehaya HaOlam": "blessed is the one who said and the world was created". We see that the world was created through speech. And as we were created in His image, and try to emulate Hashem we should also realize that our speech has great impact and is also creating. This is most likely why it is said about the ARI ZL that he seldom spoke, only on Shabbos, and only in Lashon Hakodesh. We must realize the power and effect our speech has on our children!
Conclusion: With every action and word, we are teaching our little ones and creating their world forever, therefor we should attempt to be positive and teach them with positive examples.