I'd start with the basics of 0-9 of the Major System and then introduce the method of loci. Once they've got those two basics down reasonably I'd expand their Major system up to 99 at a minimum.

The tougher part then is expanding your pedagogy to build these tools into the curriculum so that you're actively using them with your content.

You might appreciate the experience from Lynne Kelly here: https://www.lynnekelly.com.au/?p=4794. Her excellent book Memory Craft also has some interesting examples and stories for children including the use of what she calls rapscallions for use in multiplication tables, languages, and other educational applications. Her book also has a wealth of other methods and potential applications depending on the subjects you're teaching.

I'd love to hear your experiences as you progress with your class.