Hi, misslucy --
The object you're looking for is called a fog emitter, and it's a blue or turquoise colored globe in the buydebug menu. It was originally an Ambitions item, but is now base game, so you should definitely have it. There are a number of good youtube videos explaining how to make waterfalls with the fog emitter, and a google search for "fog emitter codes" or "fog emitter waterfall codes" should get you the text you need to input if the videos don't specify, or if you want to play around with different effects.
Generally, on most residential lots you will want to use the "short" codes for the waterfall, and you need to go into live mode to test to see if the fog emitter is placed properly. You will usually have to put it farther back than you think from where you want the waterfall to appear.
FWIW, even on the short, 1-tile-wide settings, the fog emitter waterfalls are larger that what I would consider "mini," IMHO. I have also previously mimicked a "mini" waterfall by using the dome fountains nestled into rocks, and/or the fountains that shoot out in an arc (sorry, can't remember what they're called) coming out of staggered rocks to look like they are trickling down the front of the rocks into a pond.
Hope that helps. If you have any questions, just ask.
