Once young toddlers begin to imitate sounds, expressions, words and acts etc, we must be very careful with what we say and do and also take care of whose company we leave them in. From the age of about 18 months, my child began to repeat many words that she'd hear. Whether it was something that I or other family members said or something she had heard out on the street. We must try and develop a clean atmosphere at home for the sake of our children at least and put utmost effort in to this.
From this young age we should encourage little children to speak only good words. Insha Allah we will be rewarded for every good deed that we teach our children and we'll be rewarded every time they make the effort to practise those good deeds.
Never laugh at a bad habit or expression that your child may have picked up. No matter how humorous or amusing it may be because that in itself is permission for your child to continue. Always stop them and explain nicely why it is wrong or replace the bad word with a good.
The first person from whom a child learns good manners is the parent. If a child is raised in a good Islamic home, then it would be natural for him to treat his parents respectfully.
Just as a child should be taught ritual acts of worship, he should also be taught good habits and etiquettes until they become second nature to him. Young children are very quick learners, it is not difficult to teach them good habits. In fact, there are times when I forget to read the eating and sleeping Du'aa to my 18 month old child and she would remind me. When they see the things that please us parents, our children enjoy doing those things. So remember to praise your child when they do something good.
those who have the best manners." [Abu Daawood]
Good manners are an acquired trait that must be adopted from a young age and it's our responsibility as parents to teach them.