Starting in 1767, there are numerous references in the Old Bailey online records to a clothing item called a "jam" or "jamb," often with the descriptor "children's" attached.
In one case, a woman had stolen items off of a girl in 1774, described as a jam, pin cloth, necklace, earring wires, and buckles. They are listed in detail as such: "a linen frock, value 1 s. a linen apron, value 3 d. a mock garnet necklace and a silver locket, value 1 s. a pair of silver shoe buckles, value 2 s. and a pair of gold earring wires, value 2 s." It's interesting that terminology of "jam" and "pincloth" changed to "frock" and "apron" within the same record.
As if to confuse the matter more, in 1771, Mary Hill says, "I make jams and frocks: on the twenty-fifth of June, I had made a frocks for one Mrs. Warburton, in Shoreditch; about half an hour past seven in the evening I was shewing the jam to Mrs. Warburton..." She differentiates jams and frocks and then uses the terms interchangeably.
In any case, they are described as linen, cotton, muslin, silk, worsted, and stript (striped). There is even a reference in 1774 to "jam cloth mitts." One record in 1776 specifies "a child's robe and jam." In terms of frequency, there are two mentions of "jam" in 1767, then none until 1771 when there are the most mentions of any year - six. The last reference to jams is in 1785, which could be due to less-detailed record keeping; a falling out of favor of the term or garment; or all of the above. There's never any gender differentiation mentioned.
I think I got to the bottom of this mystery with this wrapping gown in the V&A's collections with the description:
Wrapping gowns were a form of daytime clothing worn by babies and young children between about 1700 and 1800. They were loose fitting, but often worn with a sash around the waist. While a wrapping gown for an adult seems to have been some sort of nightgown, the adoption of wrapping gowns and other similar garments for children as daywear was probably influenced by Asian clothing given to the families of those who had trade links with the region. Lord Shelburne's two year old son Lord Fitmaurice had a 'jummer' (jama) of flowered gauze over blue silk in 1768.
I have since found a couple of garments on museum websites that I believe would be called "jams" in the 18th Century:
So, with a total of four possible jams identified -mystery solved?
ETA: While searching the Virginia Gazette, I found an ad placed by Jane Charleton on Oct. 15, 1772, listing among her wares: "... fine Suits of Childbed Linen, Robes, Jams, white Satin Blankets, ..."