Learning the melody is full of hidden pitfalls. It is well worth while checking out the backing tracks for the productions (if you are using them) before you commit. If the production comes with a pre-recorded CD with all the songs, it is so much better if the songs are sung by a reasonably well-trained children’s choir. “Out of the Ark” and “Starshine” productions have got this one absolutely nailed. They also have sensible licensing and photocopying arrangements which positively encourage you to be legal and law-abiding regarding copyright.
It is best if you are able to learn the songs yourself, and teach melody lines without relying on the CD all the time. The problem with relying on the CDs is that the children often learn a slightly different variation of the tune, and then it is a huge job to fix it later, whereas if you get them singing the correct tune to start with, you may avoid this situation altogether.
Use some kind of vocal warm up before each session. The “vocal chords” are actually a muscle, and you should do some kind of stretching exercises, just as in sport, before you start trying to be too athletic. You may already have a selection of warmups; if your school is a member of the www.singup.org website, then look no further. Otherwise, there are CDs of suitable songs: I like the Chicken Tikka Collection and have used them with children and adults.
Pluck up your courage, and teach the melodies a line at a time, “listen and copy” style, checking to make sure that the children are copying correctly. Start with short phrases and link them together. Sometimes it may be necessary to focus on just a word or two where there is a problem. It is a really good idea to use hand gestures to indicate the direction of the pitch.
Laying slow and steady foundations at the beginning pays dividends later.
Once the children have learned each song, you can play it at odd moments in the lessons to reinforce the melody and practice the words.