Hi M. - It's probably time to give your pediatrician a call. Normally a child with diarrhea lasting longer than 72 hours should be evaluated, especially for dehydration. They may also request that you do a stool sample.
You might also note for the doc and tell him the consistency, if you notice any blood in the stool, if there is a really foul odor or if there are any color changes ie: green, black, white, etc.
Continue with the brat diet - focus less on the sugars like with bananas and applesauce and more on the starches - toast, pasta, rice, etc. Dont forget to really push those fluids especially the Pedialyte. My kids liked the frozen Pedialyte pops. Again, watch the amount of sugary drinks like Gatorade and juices. Also dont use any of the vitamin-waters as those aren't recommended for children. Also give your child at least one serving of low-sugar live culture yogurt every day to help replenish the bowel flora.
I hope your little one feels better!