I recently posted about this mid-19th century toddler's outfit, and now have added info to share.
Addendum 1. Quilt historian Martha Spark got all excited about the outfit. Here's her response:
I would agree with you that the cut and construction of the dress looks to be c1860s, with its plaid fabric (the era of “all things plaid” after Queen Victoria & Prince Albert acquired Balmoral Castle in 1852) and the GREAT document fabric inside the sleeves (looks to be a Vermicular print, popular in quilts the 1840s-50s). Another fun little note on the fiber content of the plaid you so cleverly identified (wool & cotton) can be called delaine, a semi-translation of the French, mousseline de laine, i.e. woolen muslin.
Addendum 2. The next day, I came across a post on instagram from a button collector "buttondowndesigns". She's on instagram, web, and facebook. She showed a set of calico china buttons, one of which is exactly the button on the little dress! She says the buttons date from 1840s and 1850s!!! So I googled - they are a specific type of china button called calico buttons after their delicate patterning. There were 300+ calico-pattern designs manufactured, and collectors are all about trying to find them all. How cool!
For more info on these delightful little buttons:
The China Button Exchange
china button books and resources, and general button history