When discussing Estuaries with your children, the concept of a "halocline" may arise, but truly be hard for many children to picture or understand with just a definition. Even if it does not arise in the study work presented to your child, this particular noun can feel to a young elementary child like a "big kid word" they can understand and impressively define for others. Since it is easy to understand with a pretty simple demonstration, I recommend diving right in and giving your kid the chance.
A halocline occurs where salt water and freshwater are coming together but not really being mixed well. The halocline is a rapid change in salt content as one moves vertically through the water column. Haloclines have many causes and all kinds of things can play a part in whether one exists in a certain area or how deeply this "line" occurs. The important thing is just to help your child understand what a halocline is.
This simple activity is pretty cool and will help clarify the term on a very basic level for your little naturalist or scientist. Simply mix plenty of sea salt and food coloring in a container that is clear. An old Jar such as one for peanut butter would work well, as does the glass brownie pan you see above.
This colored salt water represents the ocean water.
Fill a measuring cup or something with a lip for pouring with fresh water and an opposing color to your ocean water color. As you slowly pour your freshwater into your ocean (as a river often runs into the ocean in shallows such as estuaries) you will see a line form when the container is viewed from the side. This line is where you have created a mini halocline in your container. If one does not form, you may need to pour more slowly, carefully or from a lesser height above the surface of the "ocean" water. Simply adjust and try again.
Haloclines happen because the density of salt water is generally greater than freshwater so in areas where there isn't a lot of mixing (wave action and moving currents are at a minimum) a halocline can form. Temperature has a big impact on haloclines and their stability because temperature directly effects density of all substances including both saltwater and fresh. Because of this, sharp haloclines are commonly found in fjords, still estuaries, caves, and places where the water is either still and/or very cold (Arctic ocean and the Antarctic).