I know that others have mentioned the Seaquarium but I didn't notice anyone mention that you could snorkel as well as scuba there (without being certified). I have asthma and they turned me down for the scuba so I went snorkeling with our neighbor's daughter while they did the scuba. You can get a snorkeling vest so it would be perfectly safe for your little one as long as they don't have rules against it. I don't recall any age limitations on the snorkeling-I think the age is 12 for scuba. In any case, the divers are in only 12 feet of water and it is a great way to tell if you are interested in getting certified for scuba. My husband, son and his buddy loved it while my neighbor and her husband hated it and ended up with us snorkelers. I don't think that the excursion was all that expensive and it was the highlight of the guys' trip to try scuba.
They also have something called a bubble watcher for this excursion so that you could take the 5 year old down in a viewing area to watch the divers. I did this the first time we went there and it was pretty cool. I could take pictures of my husband and son while they were doing their dive. There are barricaded areas with sharks and sea turtles as well as many different types of fish including stingrays and whether you snorkel or dive you are given a container of fish food so you can hand feed the fish. If you do this, do it first thing in the morning so the water is clear--and if you snorkel, get out there asap because it is much better before the divers go out. They have to do a training session so you have time to beat them out there.They stir up the water and the fish are lured toward them once they are in the water. They swim below you so you will be looking down at them. Neat if you have an underwater camera.
If you take the ship's excursion, they will create a tape of your dive that includes some pretty cheesy footage of your particular ship pulling into the port and then they proceed to show the entire dive. Everyone ends up on the video. For us it was a must have. You just tell them what ship you are on and they deliver it to the ship before you leave port. Unfortunately, only the divers end up on the video. I would ask them to get your daughter if she snorkels though. They may be able to do it. The cost of the tape was around $35 and it is one of our favorite souvenirs.
Just an FYI, there is a beach that you can go to as part of this excursion that is OK. Topless sunbathing is allowed.
Shopping is OK on Curacao. Nothing spectacular either way but the city is so colorful that it is fun to wander. There is a moveable bridge that is great to watch when the ship is leaving the port just be careful that you plan a little extra time if you cross it in case they move it when it is time to get back onto the ship.
We have been to Curacao twice and I loved it both times. Have fun.