Fashion trimmings returned with a vengeance in the Romantic era. Profuse decoration in the form of ruffles, flounces, and fur trims with the waistline moving down in the same ways as adult dress had, was topped by ornate hats and bonnets. The gigot sleeves of adult women were repeated in the styles of girl's dresses.
The skirt length on an individual child was a sign of her age. This lovely fashion plate right is from a book sold by an antiquarian Fashion Books bookseller Jon Edgson at eBay. This image shows children of 1837 and you can see how closely these children look like mini adults of the era.
As children's clothing has evolved over time, so too has kids costumes as you can see here. Today there are many options for consumers to choose when it comes to costumes for children. One great source is Halloween Express.