you had agreed to take someone to a horse show months ago, then a week before show been invited to a christening (not close relative). Which would you go to...
Horse show - no question! If they're not close then it won't really matter and it's not like the kid will remember either. You had arranged the horse show first too! Have an awesome time!
I'm afraid I have to disagree. A christening only happens once in a lifetime. If it was me I would have tried to get you a lift with someone else, or at least paid you back your entry fees so you weren't out of pocket.