Depending on the church and the vicar this could be a doddle or a nightmare,liaise with the vicar beforehand to find out what he will allow,as "Official" tog he may allow just you to take shots during the actual Christening itself but be prepared for him to say only set up shots after the service.
Check out the church as well because many are really dark and I can guarantee it will be no flash during the service so if you have a fast lens you will need it