I am "godfather" to some very close friends' DS, who is a little younger than my own DS.
I put it in inverted commas because, as a godless heathen (and proud of it) I wasn't allowed to be a godfather at the christening but was announced as one after (which was fine with me).
What sort of thing do you expect your children's godparents to do, if they are not providing a "religious/spiritual" context? I always give him a birthday present and play games with him and all the usual stuff. Our children are quite close anyway, so we'd probably be doing all that stuff even if I wasn't a godfather.