There are actually 3 drops
first one is just after the lift, you drop down 1 or 2 feet, and you'll probably not even notice that. This is not a drop per se, it's the boat settling down on the "water track"
second drop is just after the prison cells scene, it lasts about 2-3 seconds
third drop is at the end of the battle/fire scene. It lasts about 3 seconds. At the bottom of the drop is the photograph spot. So don't forget to put your arms in the air (if you're not holding a child

) and shout. At the end of the ride, you'll find a set of screens with the inride photographs. Budget tip : if you don't want to buy the photograph but want a keepsake, have your camera ready, and you might be able to take a photograph of the screen. Remember to disable your flash as it would render the picture useless (but actually you flash should already have been disabled since it's forbidden to use flash on dark rides like POTC or IASW - even though most guests seem rude enough to ignore this part of the park's regulations)
The drops are really easy going, of course if you hold the little one in your lap, you'll need to be careful, but there is nothing to worry about.
Remember that kids ride in the middle of the row of seats (especially younger ones)
The most "dangerous" part of the ride is at the very end, just before unloading. Very often there will be a "traffic jam" of boats waiting to unload. So you'll most probably bump into the boat before you, and sometimes it's quite hefty. But what you need to watch out for is the boat behind you, as she will hit you boat hard from the rear. So do not let go of the kid in your lap just because the drops are over and you're at a stand still. Wait and remain seated until you're instructed to disembark.
You may occasionaly get wet (very lightly) if you sit in the front row at the bottom of the drops, but that's only a few drops of water, by no means will you get soaked.
DS did this ride so many times since he was 2 1/2. Sometimes younger ones are freaked by the skeletons at the end of the ride, or the sound of gunshot and swords, but in the end they eventually like it.