I'm unsure about removing the mechanisms of the talking plushes...they never seem to work right and children tend to freak out a little to see pinkie's guts on the kitchen table
I've found that the Resolve carpet stain remover and a good scrub with a rough washclotch does a pretty good job. Spray the cleaner on the wet cloth and not directly onto the pony, though. Oversaturation may cause it to soak into the cotton inside and your child may have an allergic reaction to it. After the initial scub, use some clean cloths to get rid of any remaining soap from the plush. Don't soak them, just lightly damp. It may take 4 or 5 cloths to remove all the cleanser, but they smell and look wonderful when you're done! I find using white or pale cloths work best so you can see how much dirt is still coming up off the plush.
If it comes to the point of having to wash by removing the talking mechanism, try doing it when your kid is asleep. After taking out the mechnism, put the pony(ies) into individual pillowcases when you toss them in the wash. Front loader works best. Remove it after its bath to check how clean it is and repeat as needed. You may have to replace the stuffing all together depending, but you'll know how far to go as far as that's cancerned when you see inside.
After she's well washed, keep her in the pillowcase and toss her in the drier under low heat. It'll keep the plush from possibly melting or going super pilly. When you pull her out, let her sit and continue to air dry for a day or so...she may feel dry when she first comes out of the drier, but my experience is that air circulation either outside in the sun or just in the laundry room is the best way to ensure mold doesn't grow in some of the tight areas like the seams and hooves
Good luck, and i hope you can get your daughter's new friends can be brought back from the brink!