singing-start with the upper register exercises-sirens to get kids ears hearing the music correctly and singing in tune.
When you start singing, choose a group of 5 or 6 kids to sing without the rest of the group so that you can tell if they are singing correctly or not
Don't sing in a whiny bad voice if you wouldn't be willing to do it in front of the whole class or whole school.
It's not what you're singing, it's how well you do it and how much fun you have with it.
If the kids sound good, maybe they will like singing more.