Rather than a christening, because neither of us practise religion, we had a Godparents Day. Got a friend who's a civil celebrant to come along and basically formalise our son's godfather and godmother in the presence of all our friends and family. We wrote some words about why we chose these people as godparents, and they spoke about what the role meant to them. It was a great day. Fun, funny, and moving.
I know the 'god' word is still in there, but he/she wasn't referred to in any more but the title of the role. And mentor sounded a bit stiff and unloving!