Psychology and grammar

Children always try to generalize things. For example, we can see that children often try to do things like the elders did. They don’t know the consequences of such things. They also may not think about the dangers or mistakes also when they try to imitate others. They learn lessons from others and from their surroundings. For them, if two or more people do a thing, that is right. Same way when they hear more words with ed extension for representing past tense, they will try to generalize it.
In short, children use ed extension with every word they used because of their increased awareness about the need for grammar in language even if it is wrong at times.