I have done a few christenings now and everyone has been stricter than a wedding due to the more religious aspect of the event. I was only allowed to shoot a few shots during the actual christening with no flash allowed or af beep. Typically the shurch was badly lite and requires my Canon 50mm prime wide open at f/1.8 with iso ramped up to 1600 just to get a few shots in the church during. Most of the shots were after at the party.
My advice would be get some fast glass, and speak to the person who is in charge of the service, see what their wishes are, one other thing you will have to remember that unlike a wedding where 95% of the people in the church are their for that service only and expecting to see a wedding tog, at a christening it is performed as part of a normal Sunday communion so it will be filled with members of the general public and you will have to respect their wishes too.