Mix some pedialyte with the formula. 1/3 pedialyte and 2/3 formula (normally mixed formula). This will loosen the stool and make sure the kitten gets enough fluids. When the stool is normal you can try only formula again (but the kitten formula from RC at least in Sweden has a bit of a reputation to cause constipation).

You can also put a few drops of paraffin oil in the forumla and on the anus. This lubricates the intestines and anus and makes it easier to pass on the stool. Paraffin oil shouldn't be given more than just a short period since it also affects the nutritional uptake negatively.