A little is a person in a BDSM relationship who most often submits, either through their actions in a BDSM scene or by means of a power exchange.

Little girl is an ABDL term and is related to the DD/Lg path. This is a relationship that usually involves an older and younger adult involved. Boys can be Little Boys and works the same way as the girls do

Littles can range from child-like to childish both in appearance and their behavior. Contrary to what most people think, BDSM littles can come in every shape, size, race, ethnicity, sex, gender and mindset. 

One important thing to note in any aspect of BDSM, including littles, is that only consensual adults of legal age are concerned.

Baby girl is an aesthetic taken from the DD/LG relationship which is a  BDSM subgenre. Being a babygirl involves exhibiting sweet and innocent imagery and attitudes that makes the adult have an overall childlike feel. A baby girl is usually a late teen or 20+ of age. They act girly, immature and also bratty.

In order for a baby girl to truly be herself with her Daddy, it is important for her to find someone she trusts completely and without hesitation. This trust should start outside of the ‘dungeon’. Her trust must be unwavering and her daddy must not break the trust. This is important for the relationship to be fulfilling for both partners.

To me, littles are a subset of the submission part of a BDSM relationship. However, being a “little” does not mean that’s all of who you are, though.

This is because you can be both a submissive serving  your dominant partner and a BDSM little who needs or wants to be taken care of by your Caregiver/Dominant partner. However, the point is, both submissive dynamics cede some level of control to a dominant partner.

MarlyMonster on Reddit speaks on what a little is;

"...Being a little is about allowing yourself to expose that most vulnerable inner side of you and giving yourself over to your caregiver. They’re entrusted with a very fragile part of yourself, which in turn is something a caregiver lives for. They crave to care for someone, and a little craves to be cared for…."

But all “littles” express themselves in a personal way that is infinite and unique to the individual.

On one end of the submission spectrum, a baby girl is one who acts younger than her chronological age but doesn't necessarily identify with any specific age. They are used by those who have daddy dominants and have a childish/bratty kink. A baby girl may enjoy things related to children such as Animated/Disney movies, some stuffed animals, and earning gold stars or not. It is also used by vanilla couples as a term of endearment.

At the other end of the spectrum are people who identify very strongly with a specific age and exhibit the attributes of that age. This includes (but not limited to) diapers, sippy cups, toys, pacifiers or whatever they deem perfect for their “little space” age. For some they exhibit these behaviors only in the bedroom. Often, there is a different nickname for the Little.

Some may identify with very young ages while others may feel more like teenagers. This means that the particular accessories and activities they associate with will vary. Sometimes, BDSM littles only identify with an age range while others can specifically state an exact number.

Anyone can fit in that spectrum from one end to the other and there is a lot of variation. Within that spectrum, there are people who specifically want to be part of a Caregiver/little dynamic as a relationship style. Others may be Adult Baby/Diaper Lovers. It’s completely possible to enjoy both. All are valid as long as it’s consensual.

Here's what Maxx Velocity on Quora thinks of both terms;

"The terms are somewhat interchangeable in BDSM but a Little is usually someone with a very decided difference between their adult personality and their childish one. Baby Girls often do not engage in age-regression, but will flirt with and seduce their daddy with childish behavior incorporated into their adult persona."

Both terms are very much related as they are both subsets of the submission spectrum in a BDSM relationship. However, there are some distinct and unique characteristics that distinguish a BDSM Little from a Baby Girl. Be yourself and own whichever you identify with wholeheartedly. Good luck.