Raw, unfiltered apple cider vinegar will work wonders for a sour crop. Basically, a sour crop is because the good bacteria is out of whack in the crop. Many things can cause this, from antibiotics, having a yeast infection, fungal infection, something the chicken ate, or something internally wrong with the chicken.
But more times than not, it comes from an imbalance of these good bacterias. Yeasts and fungus grow in these environments in the crop, in a sweet environment. Apple cider vinegar lowers this ph of the crop to an acid environment and does not allow these yeasts and fungus's to reproduce.
Sour crop is difficult to treat and it does not disappear rather quickly. So you must keep up with it. What I suggest you do is put 3 tablespoons of this raw ACV to one gallon of water. Give it to all your chickens. It will do their systems good too. I offer ACV one a week for good health of my chickens. Change the water everyday and keep adding the ACV for 2 weeks.
If she is uncomfortable and is doing the "neck squiggle" you might want to vomit her to get all that toxic stuff out of her crop. Hold her like a football, and lean her forward, never backwards. With the other hand, massage her crop till the gunk comes out her beak. ONLY hold her like this for no more than 10 secs so she can breathe. Do it one more time to get the rest of it out. You won't get it all, but the worst of the toxic stuff will come out and she will feel better. Do this twice a day or more if she needs it.
Keep offering her yogurt, hard boiled eggs and soft foods. Wet her feed down. Don't offer any hard foods like raw veggies, seeds and such. You want everything to move as fast thru her system as it can, as a soured crop is extremely slow to begin with and you don't want to slow it further.
And, if she appears cold in the mornings, you might want to consider keeping her under a heat lamp at night. Cold birds do not move their crops or heal from illness.
Do all this stuff for a few weeks. It takes a while to get her back on track. I had a girl with sour crop for 2 straight months. But she completely recovered and I use the ACV once a week to prevent any sweet crops in the flock.
Good luck with your baby!