Rhythm can be a very abstract concept to grasp, especially for young students. When teaching students new rhythms, mnemonic devices can be very useful. The easier to remember the better, and for young students, that usually means the simpler, the better.
A great way to keep things simple is to use the names of fruit or other foods. For instance, the word grape is a one syllable word, which takes up the amount of space equivalent to a quarter note. Apple is a two syllable word comprised of two shorter sounds. When spoken, the word apple sounds like the rhythm of two consecutive eighth notes.
Here is a chart that shows a few more advanced examples: