I, myself, have never gone down the slide, but, I have seen little children slide down it, and they seem to enjoy it.
They are usually fairly strict about adhereing to the size limits for going down the slide; therefore, to prevent your child from being disappointed, I would measure them before they left home. Because I have seen children sent back down the steps, without going down the slide, because they were too tall or too short.
Look at this way, if the child is too short for Mickey's Slide this cruise, you have another reason to go on a future cruise. Enjoy!