Can you try and find "Pebbles" (Longman)? They have the karaoke version on the cassette. I have it, but not on a cd ... Now, let me take a look at the other CD �s i have and see if there are karaoke versions ...
EDIT: Yep, i have it. Will upload it soon and send you the link here.
EDIT again: Seems there isn�t a karaoke version. Other songs do have it on this cd, but not the wheels ... Bah! Will do more search.
EDIT once more: I take it your ss are little, probably not older than 8 or 9, so i would suggest that you keep the original version and make the ss sing louder. It happens oh so very often that they don´t listen to the beat and sing faster and faster, OR that they get very shy and need a boost from you (while standing on the side) to help them out, OR ... There�s nothing wrong with going with the original song. I do performances a lot and i find that karaoke versions can be a failure at times, especially with the little ones.

Oh, and another suggestion. If you have the time, try to ´get´ the people from the song onto the ´bus´ using very simple vocabulary (depending on the age of the ss). That way you will have a short play, too, and amaze the parents even more!