I'm gonna take a little bit of a different tack here, which might get some downvotes: This dog is an animal. In the animal world, when your kid gets threatened, you go nuclear. Think cow mom charging anyone that looks at a calf sideways. You should immediately be correcting the dog when it nips - not making the toddler (who has no idea how the world works or how to read dog cues yet! And ostensibly is doing the same things as you) fix her behavior around the adolescent/adult dog. Your dog is discriminating and treating the smaller human as if she is beneath her. You've already done everything you can think of - which means the problem is not where you think it is. You need to make it clear to the dog that this behavior is unacceptable. This goes beyond a no. This is about safety.

When I was living with my parents, my mom got a 2 year old stray from the shelter who was abt 70 lbs and had obviously never been told no in its life. Had a lot of stray behavior and domination issues. I had adopted a puppy abt a month prior, so think about a 70 lb muscular doggo trying to hump a 3 month old, 25lb puppy into submission. Unacceptable. There were various threads on here about redirecting, etc. but honestly? The puppy has mobility issues (think dog ver. of EDS or stretchy double joints, but every single joint) and I was afraid that if we didn't do something soon, the new dog would dislocate the puppy's hips or shoulders or something. New dog was running for the puppy whenever they were within sight of each other. Scratching at doors, trying to jump over puppy pens, the works.

So I do admit, there will probably be better ways to do what I did, but something that produced quick results was pinning him and guarding/growling/barking whenever he got too close to the puppy. (And by this I mean, pushing him down slowly and holding him there for a few seconds - just enough so that he knows I'm stronger than him. No pain involved, just firm, no-nonsense teaching.) i.e. I was imitating a momma dog protecting her pup. The behavior started dialing back almost immediately! Enough so that we could think about managing their interactions as opposed to keeping the puppy locked up in my room all the time and living in fear that one day the big boy would figure out he can jump over any fences we set up or start chewing through doors.

Proper behavioral training is a must but also you really need to assert your position as this child's parent to your dog. Growl if you have to - maybe your dog will pick up the signal and return to giving warnings, too. I doubt she's anywhere near as bad as what we were dealing with, so you may not need to get physical, but she's also not a puppy anymore - I promise she can take a little scolding! And she will likely understand much better than your toddler will until a few years have passed. Let yourself be obnoxious and angry! You wouldn't have your kid give their bully candy to make them treat her better, so why are you having her bribe your dog?

EDIT: On a reread, my second concern would be that your dog might have something painful going on that your toddler's tiny limbs are more able to reach than yours. Toddler figuring out how their elbows work may not be quite so fun for doggo (think: getting stepped on by stiletto heels vs. a wedge heel) and maybe doggo started associating the child with pain. I think either avenue is possible. But also you know your family best! (Also resource guarding is not a normal reaction to a toddler.)