We went to both parks last year. Our kids were 13, 10 and 5. We love Tampa much better. It just seems so much bigger and has tons of animals. There are plenty of rides for the younger child in both parks but I actually thought there were more in Tampa. There is also a huge play area with climbing structure in Tampa. I can't remember one in Williamsburg. There is one in W'burg. It's called Land of Dragons. Plus, you can feed the lorikeets which has always thrilled my children. You can do this in W'burg too. If you time it right, you might even be able to feed the huge tortoises bananas on a stick.
We noticed that there always seemed to be a small ride located next to a larger one which made it easy for our older children to ride the coasters while their little sister had something to do. I, honestly, don't think you can go wrong with either park.
However, if you have older children that like history than I would definitely say take the summer trip and do the triangle package (Busch Gardens, their waterpark and Colonial Williamsburg). It was cheap, under $130 each. My older children can't wait to get back to Colonial Williamsburg but would be happy to skip BG Williamsburg the next time around!
Have fun!