After my grandson visited and it was raining (he is 20 months) I asked my daughter if he had a raincoat.  The answer being NO and this being the Pacific Northwest, I quickly dropped into Amazon and found this amazing little raincoat at a great price.
He went for his first walk in the coat and even stood under a down spout and emerged completely dry.  My daughter put it over his regular coat and she said the hood kept the hood of his coat up (which normally fell down).  The sleeves are long and kept his clothes completely dry and the length was Ok for a 20 month old at the 3-4 size.  It could even last him a couple of years.  And it is nice looking.