Pumpkin or Butternut Squash purreed you could try roughly 1/16 of a tsp - most measuring spoons have 1/8 tsp just divide that in half since Charlie is just a wee one.
A little bit of ground flaxseed will also lubricate the poop to make it for her to go. Put a heaping tsp of ground flaxseed into a glass dish, add 1-1.5 tsp hot water to make it slimy then add the tip of the teaspoon a mm or 2 deep she shouldn't need more than just a wee bit.
Are the kittens drinking enough water?
Avoid biting when a simple growl will do
The Spirit Lives As Long As Someone Who Lives Remembers You - Navaho Saying
Vindication ~ For all those pets who became sick or lost their lives from tainted pet food