My wife (girlfriend at the time) and I went to both islands 7 or 8 years ago. She was a non-diver when we were in Curacao and a just certified diver when we were in GC. Both places had plenty of shopping (they're both popular cruise ship destinations). Curacao probably has better diving, unless you head out to the East End of Grand Cayman.
The Curacao seaquarium isn't a huge, impressive aquarium with all sorts of exotic species. It's a series of small tanks with local (but interesting) fish, eels, etc. They do have a few petting pools, in case your daughter wants to play with a conch or sea cucumber.
GC has much better beaches - the shore in Curacao is mostly rocky, with a few manufactured beaches at the larger resorts.
If you think your daughter is old enough to try snorkeling, GC is probably the better choice. The water off 7 mile beach is almost always calm. There are a few reefs (like cemetery reef) that are a very short swim from shore. Curacao can be a little rough by the time you get to the interesting sites.
So I'd say your wife & daughter will probably enjoy Grand Cayman more (though a 4 year old would like the seaquarium), while you'd probably like the diving in Curacao more.