Tha'ts right! , the special formula for kittens is called KMR , it has special enzimes for the fortifications of the kitties system.
They probably starved, when new born stage : they need to eat every 2 hours .
Don't feel bad about what happened and learn from your experience .
Do something good for"mom kitty" SPAYING HER
and if cost is a concern ... check your local animal organizations for low cost surgeries.
If you spay your kitty it will diminish her chances to get PYOMETRA ( Cancer in the uterus ) when older.
I worked in Animal Hospitals before and this was always a common situation on unalteded pets.