thank you.I've done what I can do,but it was still a little hard to let my students read them very fluently.Maybe they need some time.The pronunciation of these words was difficult for Chinese kids.

I suggest that you teach it with hand movements. "on the desk " Demonstrate then tell the students " Put all your hands on top of your desks" "under the desk" Demonstrate by bending down and putting a book under your desk. Then tell the students: Put your books under your desks"

hi.maybe with song or equivalent this word in your language or play spelling words

Hi..I also teach kids and the lesson today is about prepositions : on & under..in fact I didn't face any problem about how to teach them that,,try to write these two phrases on the board and let your students lesson to your pronounce to these phrases after that ask them to repeat after you..or ask them to put their fingers on the phrases in the books and repeat that.