Jeez Louise! Didn't anybody here ever take sight singing in school.
The way you teach any and every student to count, is to first close the lid on the keyboard. Then you flaten out both hands directly in front of you.
Next, tapping on the the lid, you count the long note beats in your left hand, and at the same time you count the sub-beat notes in your right hand. Usually, this mean left hand lower staff, right hand upper staff.
At the same time you count it vocally out loud, eg. one (l.h.) and two "and a" (r.h.), etc. Or, you can use "te" or "ta" instead of "and."
Once you do this for every piece you learn, you get will the feel and the sound of it in your head.
Also, if there is a Dalcroze Eurhythmics teacher in your area, take every class they offer. It will drastically improve your sense of time.