I'm an atheist, so Christening songs aren't exactly my expert field. However, on the few occasions I've been dragged into a church (usually for someone's wedding) I've always found 'Lord of the Dance' will bring a smile to everyone's face.
I've just done a bit of Googling and 'Sing Hosanna' has come up quite a few times as a suggestion which, I feel, would also sound good with a gospel choir belting it out.