One of the easily recognizable (and cutest) elements of 18th century children's clothing are leading strings. Attached to the shoulders of a child's garment, the strings could be held by a grown up to guide, stop, or assist the child when walking.
It should be noted that leading strings predate the 18th century, as shown by this Rembrandt sketch (1645):
http://www.rembrandtpainting.net/rmbrndt_selected_drawings/child_and_old_woman.htm
But to get a few examples of 18th century leading strings, we can look to extant garments:
http://www.metmuseum.org/collection/the-collection-online/search/84402?rpp=30&pg=1&ft=+1990.24&pos=1
Even extant dolls:
http://collections.vam.ac.uk/item/O96264/doll-unknown/
And fashion prints and artwork:
Jean Etienne Liotard, Girl Singing into Mirror
http://www.the-athenaeum.org/art/detail.php?ID=93132
http://mimic-of-modes.blogspot.com/2013/03/galerie-des-modes-32e-cahier-2e-figure.html
http://mimic-of-modes.blogspot.com/2013/12/galerie-des-modes-63e-cahier-3e-figure.html
Detail:
The Groenmarket by La Fargue, 1765
https://siftingthepast.files.wordpress.com/2013/02/siftingthepast_the-groenmarkt-as-seen-toward-the-westeinde_paulus-constantijn-la-fargue_1765.jpg
And even adorable porcelain figurines:
http://museum.bamberg.de/sammlung-ludwig-bamberg/ausstellungsdetail/news/glanz-des-barock-sammlung-ludwig-in-bamberg-fayence-und-porzellan.html
So where does this leave us? Leading strings were included in some 18th century children's clothing, including gowns like the gown I am working on now.
Confident Assertions:
1) Some children's clothing included leading strings. This is seen in both extants and prints.
2) In an area of research I'll call "experimental archaeology" I can say with certainty: Leading strings are exceptionally helpful for keeping a toddler in check. Danger lurks everywhere in the 18th century world, and having a firm hand on those strings? Priceless.
3) Children, or at least some children, like them. In that, given the option of being "on the leash" but able to explore and walk on their own vs being carried, it's an appealing choice.
Claims I've read or heard but don't know much about:
1) Older children (girls) retained leading strings on their gowns as an affectation. For example, Liotard's singing girl is old enough to not require a Baby Leash (or maybe that's just what I call it...). However, I have no idea what the "rules" on this were. How old? Was this a fad that faded, or fairly constant throughout the period?
2) Leading strings were for helping children walk. Some images, like Rembrandt's sketch, hint at this--the woman seems to be at least partially holding the child up by pulling up on the strings. My child learned to walk in the reenacting "off season" so we didn't experience this. However, for our experience, as nice of an idea as "walking aid" is, I've found that leading strings function for us as a cute leash. Note that some of these images combine the leading strings with the "pudding cap," indicating a kid who's still unsteady on his feet, but many images don't.
Things I don't know:
1) In plenty of extants and images, children's gowns/clothing skip the leading strings. Is there a reason/rationale for including vs not including them? Strict personal preference? We cannot write off the lack of strings on extants as "wear over time" as they are also absent in many--if not most--images, as well. Children of the same age are depicted with and without strings.
So, knowing how handy they are, I wonder--why NOT have leading strings? Getting further into when clothing did and didn't include strings is a good research question...something to dig into!
2) Material? In some, such as the extant gown from the Met, it's clear that it's the same material as the gown, but in some prints, there is a color contrast. Is this fabric, or ribbon, or tape? I would venture that it's ribbon in the image of the girl in pink walking the child in white from the included rosette on the garment. Same here--I think this is a blue ribbon attached to the clothing:
http://commons.wikimedia.org/wiki/File:Angelica_Kauffmann_tolstoyFamily.jpg
Fun questions to ponder about a cute clothing feature! And for the record, I will be including a set on the gown I'm working on now, likely in the same fabric as the gown (but in case I don't have enough, it's nice to see that I could potentially add ribbons!).