Admission to DC includes 14 days of admission to both SeaWorld and Aquatica. The SW/AQ admission can be used either before your DC day, after DC, or a combination of both

. You can also add on 14 days of admission to Busch Gardens for an additional $20 pp, if you so choose. And if you stay at a SW onsite hotel like the Renaissance, then you get complimentary transportation to BG as well.
Your kiddos would not be old enough to do the dolphin swim at DC (you have to be minimum six years old), but don't let that dissuade you. DC is an amazing park even without the dolphin swim. Remember, too, that your DC admission includes a hot buffet breakfast, buffet lunch, all your snacks, all your drinks (including alcohol), and a free 6 x 8 family photo. Parking is also free.
I have not personally stayed at the Renaissance, but the location can't be beat (its right across the street from SW). It has a great water play area for wee ones as well. For our last SW trip, we stayed at the Residence Inn which provides a free hot breakfast as one of the included perks (if you want to check out the RI, see the link in my signature to my Christmas TR). The Residence Inn also has one bedroom suites which might be nice for your family, to put the little folk to bed while you can still watch TV in the living room.
Any other questions, feel free to let me know

.