Hi Erica, I'm so saddened to hear of your loss. Please know that you're not alone in this time and feeling. Unfortunately, many of us itty-bitty kitten foster caregivers have lost a little one for some unforeseeable reason. I've been there before, and definitely know how hard it can be. BUT!!! Know you're doing a great job, you're asking questions, you're giving TLC and that those 4 other kittens and mamma are grateful to you! I'll let @Dr. Sheila Segurson weigh in on the medical, but here are a few resources top of mind for the transition process (it's super duper fun/cute btw and mom will do most of the work). In my experience the litter box transition esp. with momma present can be as easy as making freshly cleaned litter easily accessible.
-Amber
Maddie’s Fund video:
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Kittens need to be fed frequent small meals. They should be fed a minimum of 3-4 times per day. Their stomachs are smaller than adult cat stomachs, and they have higher nutritional requirements than adult cats do. We recommend that you leave dry kitten food out at all times.
For younger kittens (3-6 weeks of age) who are transitioning to solid food, it is acceptable to moisten commercial kitten foods with milk replacer or water to make a gruel. Older kitten(s) also benefit from being fed wet food several times per day, in addition to dry food. Gruel and wet food should not be left out for very long or it can spoil.
Additional information:
#PetHealthandNutrition