To break it down...
SAB for little kids - huge sandy bottom kids pool w/ tons of sand to dig in, small round kiddie pool, playset, pirate ship w/ kiddie slide and kiddie pool.
BW for little kids - small round kiddie pool, playset.
i've been to both w/ my kids who are now 8 and 6yo. we did sab when they were about 4 and 2, which was wonderful! The sandy bottom kids pool is really, really, really great! there's also a small playset and a little round kiddie pool right there too. the food counter and table and chairs are there too. we spent full days there. my kids also enjoyed the pirate ship slide/kiddie pool at sab.
When they were around 6 and 4 or maybe 7 and 5, sab didn't work as well. The sandy bottom part was too shallow for real swimming, and the other parts are fairly deep in spots. the real slide was too big, the pirate slide was too small. The boardwalk was our fav. for this age... they went on the clown slide constantly (i really enjoyed the clown slide too!) and swam in the regular pool.
The boardwalk does have a really cute kiddie area w/ a small round kiddie pool and playset, but i think for your kids ages, i'd do sab. The small round pool at bw is the same as the one at bc, plus at bc you get the sandy bottom huge pool. The digging keeps them busy for hours! AND the sandy bottom pool is great for mom and dad to sit in w/ them... not so much w/ those small round kiddie pools (ewwww!)... I feel like the huge sandy bottom pool has more water to disperse anything that might need dispersing.
I'd definitely do sab for now w/ your kids ages. you can do boardwalk when they're a little older.
enjoy whatever you decide!