scrapper - I do not know when you are planning on going, but you might want to check the park hours, especially if you are going to the MK/AK after 4pm during the week. I am not sure how late the park is open, but I remember when we were there last fall, the park closed around 7pm. Also, we stayed at the YC, and getting to the MK does take time, given the bus schedules, so if you do go after 4pm, be sure to figure in some bus time (we did not rent a car, so I do not know how long it takes to drive, park, etc.) I do not know if you are locked into staying at the BC, but in your situation (solo during the day with three kids, and a desire to start your day at the theme parks after 4pm, the Poly might be a better choice. Kids love it, and transportation is the best.)
If you stick with the BC, Stormalong Bay is wonderful, and I am sure that your 4 and 9 year olds will enjoy it. The pool is quite large, so you might want to strategize with some YC/BC veterans on how they keep track of their older kids with an infant. Good luck and have a great trip!