I teach middle school, but I have also taught elementary music as well, and junior high as well.
I know this will sound strange, but I used a piece with 7th graders one year, and tried it out on 2nd graders the following year, and they did just as well! It was called "Just One Candle", (it can be purchased from musick8.com), or you could listen to pieces of it, if you would like. I added sign language, and the students did wonderfully.
Some of the signs are the same on some of the words, so that helps.
And, the parents were quite impressed that 2nd grade would know sign language!
It is not a religious song, the theme behind the song is peace on earth.
Or, it would be really pretty, if you could find some of those candles that are battery operated, and have the students hold these while singing. And, for effect, you could have them light their candles one at a time throughout the song.
P.S. I haven't been on here in a while, sorry I'm just now seeing this.