This is a question I've never seen addressed in any of the toddler health books! We struggled with it too, and still do.
I practice with my daughter whenever it seems relevant to do so. I tell her it's like blowing out a birthday candle -- with breaths from her nose! So we do "huff huff huff -- ha ha ha! huff huff huff -- ha ha ha!"
And then most of the time she still sniffs/inhales with the Kleenex. :o But she's getting the idea!!
Don't worry about the tissues and the nose. Eventually kids do figure this out.
If you've got sniffles and sneezes, try Hyland's homeopathic Sniffles and Sneezes remedy. You can find it at Whole Foods and IMO it's worth the money; provides relief within 15 minutes.
Best of luck! Keep teaching, and have fun with it!