Mr. J. L. Bell, on his Boston, 1775 site has written an article of interest to this discussion of stays entitled "The Ruffles so Shockingly Hemmed" which is a tale from a children's book that references as a reward a new pair of children's stays for improved behavior on the part of a little girl. Not something one would think would be included in a reward system for children!
As noted in many of the advertisements children's stays were widely available and for sale here in the colonies and there are children's stays in several museum collections.
The Museum of Costume in Bath has in their collection a pair of children's stays. Follow the link for the complete and detailed record.
The Musuem of Costume estimates these stays would fit a two year old child. Notice that these stays do not have waist tabs as the young child's body does not yet have defined hips, on an adult the tabs flare out over the hips, preventing the boning from digging in to the wearer and also providing strength to the entire stays.
The directions of the boning channels are vertical at center front and only slightly slanted at the sides of the body. The purpose of these stays is to provide support and protection and not to force the waist into a narrow shape. These stays appear to be in only three pieces, one center front and two side/back pieces and are constructed of only two layers of linen.