As I mentioned in my past post, we've done the 4N Eastern and the 7N Western. Notwithstanding the fact that 7 nights are better than 4 and I may not be comparing apples to apples here, but I would opt for the Western myself.
The Western ports-of-call were great for kids of any age (mine are 10 and 7). Some examples of what we did:
Key West has a neat shipwreck museum and a cool little aquarium (their 'touch tank' was a huge hit with my kids). We also took a trolley tour of the town that was interesting for whole family (the kids even got souvenier coins from the trolley driver). There is shopping there, but we're not too much into shopping except for souveniers.
Grand Cayman has a turtle farm that was pretty enjoyable. The kids even got to hold sea turtles. As well, we did the Stingray City Snorkel Tour which is not to missed. Very cool for the kids and parents alike.
Cozumel has a ton of excursions that we would have liked to do, but the age of our kids was limiting. In the end, we took the excursion to Chankanaab national park. It was well worth it for us. There's just a lot of stuff to do there, including a sea lion show (unlike any I have seen before), dolphin encounter, Mayan 'jungle' with a recreated village, etc., a beach, and lots more. The cab ride to/from the park can be worth the price of admission....
And, of course,
Castaway Cay is a highlight of any cruise route. I urge you to take full advantage of everything there - including bike rentals and snorkelling. My youngest (7 year old) did the snorkelling reef there for the first time ever and had a blast. Hint: get a disposable underwater camera and shake the rubber strap under water. You'll get loads of fish to follow you! (The camera is also great for Stingray City if you go...).
Also, the 2 days at sea during the Western are great for resting up from excursion fatigue. Of course, on those days we tended to take advantage of some of the ship's entertainment and educational offerings (e.g. they ran a lecture series on how the ship was built, themed, etc. that is not to be missed).
To finish up this long-winded note.... The Eastern cruise we took stopped at Nassau and Castaway Cay. Nassau just did not impress us vety much. We took the harbour boat tour, which was okay but not a highlight. We are not shopaholics and were just really turned off by the Straw Market and plethora of jewelry stores. Of course, other Eastern ports-of-call may be way better, but I can't tell you anything first-hand.
Hope this helps!!
Kevin