Sylvia is probably getting a poopy bum from the kind of food she's getting. Baby rabbits should only have plain alfalfa pellets and a dry grass hay. You shouldn't feed bunnies veggies until they are 12 weeks old. Then when thay are that old you start giving one veggie at a time and only a very small piece to make sure they don't get soft poops. If they get soft poops, you don't feed that veggie any more until they are older, then you can try introducing it again. And carrots aren't considered a veggie but more of a treat and they should only be getting a very small piece when they are old enough to have veggies, and only if it doesn't cause soft poops. They also don't need to be getting oats or vitamins in their water. There are already vitamins added to their pellet food.
Almost every time a rabbit gets soft poops, it is because of something they are eating. This is what you want to do. Make sure you are picking everything out of the vitakraft mix and giving only the green hay pellets to your rabbits. Stop giving all veggies or fruit if your rabbits aren't old enough yet. If they are old enough for veggies then just stop giving them to Sylvia. Stop giving the oats and the vitamins in the water. Give Sylvia only a small amount of the plain hay pellets and lots of dry grass hay, not fresh grass. Give her as much dry grass hay as she wants to eat in a day so that she never runs out of it. This should get rid of the poopy bum. You can use a warm damp cloth to clean off her poopy bum, then make sure to towel dry her fur as much as you can, and make sure she is warm enough until she is completely dry.
When you can, switch them both to a new rabbit pellet that is plain with no seeds, fruit, grain, or colorful bits in it. You need to do it very gradually by starting to add a very small amount of the new food, like 1/2 teaspoon, into the old food, and each day you add just a pinch more of the new food, and a pinch less of the old food, until after 2-3 weeks you are all the way switched to the new food.