It's easy to forget as an adult that learning new words can be really hard. There are more than a million words in the English language alone, and most of them we don't even use all that often. Add to the mix a confusing plethora of accents, contexts and associations and a kid just learning to string together sentences doesn't stand a chance.
Language acquisition is a fluid thing, and sometimes when we get things wrong we make them a little better than they were before. Other times, they make no sense whatsoever. Kids being kids, usually they fall into the latter category. At least it keeps things entertaining.