I have helped a lot of people with digestive issues, including children. I have a friend whose child had severe encopresis all the way through high school - she did the gastroenterologist and the behavioral therapist, and it helped some, but even at age 13 her daughter was having the problems yours is (soiling before bowel movements, urgency, multiple accidents, and so on). It's a long road and a difficult one, not to mention very expensive.
I would question the fiber gummies - if you put most gummy vitamins in a dish of vinegar for 20 minutes, see how much is left. (probably most of it!) how much is left in the child's stomach after the customary 20 minutes food spends in there. The rest is likely wasted. Gummies are a great marketing gimmick but hardly anything in a regular pill form has more than 25% absorbency (often less), and the gummies are much tougher to digest.
A good comprehensive supplement with soluble and insoluble fiber (almost impossible to find), with a patent, and with many years of clinical data is what you need. Comprehensive means digestive herbs and enzymes on top of the 2 types of fiber combined. This can help heal the digestive tract and, for all of my clients who use it properly and consistently, has enabled them to get rid of all their symptoms of IBS, constipation, diarrhea, colitis, diverticulitis, acid reflux and similar conditions. This is not in pill form and can be added to water or any liquid she will drink. If you don't know of a source for this, I can help guide you.