Most of the Halloween stuff is at night. The lines will be very very very long & very likely you will question if it is worth the wait. I can tell you that I have been pretty impressed in years past and have not regretted the long lines.
The haunted house stuff can be pretty scary, but I've seen pretty young kids handling it ok.
I'd get their early and hit the big rides. The food for the most part is not all that impressive, except that the BBQ is pretty decent.
I really think you have one day to many planned for Busch Gardens. It is not that big of a park. Unless you plan on standing in the hour plus lines for the big rides that will develop, you might want to make other plans for the last day.