Hi Lee,
One of the many things to consider is churches can be dark if it's overcast. Your 50mm may be a good idea, but the 18-105 may be better if you want some group shots after the service.
It's always a good idea to check with the priest/ officiator if you can take shots during the service, as quite a few don't like it. Your niece is being welcomed into the Church and it is something they like to concentrate on rather than having cameras going off left, right and centre spoiling the occasion.
I can remember when my niece was Christened last year, I got asked to take some photos 5 minutes before the service started! Never done one before either!
If mum and dad have any other kids, make sure you get them in some pictures too. I also took quite a few of all the invited kids with mum/dad/baby in a group as well as the church candle, Christening candle, the font and outside the church.
The church you can shoot anytime up to (and probably just after) the event, so it saves you having to worry about it on the day.
Depending on the weather on Sunday (I don't know where you are), it's probably going to be overcast. Think of what you'd normally do for taking shots of people. Just because you're indoors doesn't make it that much different. Position yourself between a window and them (like the sun at your back) if you can, unless you're good at controlling light.
Some of the best shots I got of my niece were actually after the Christening in the church hall as the light was better (and I only had my kit lens on me that day), but my sister-in-laws favourite non-person picture was the church candle. Sometimes the little things get missed during the big day.
I notice you haven't had much use of your 50mm yet. I got mine late last year and apart from a few shots, it hasn't left the camera. My Canon is great little lens, but my first few shots were rubbish. The best advice I can give on that is to play tomorrow with it and see what you get. Line up a load of things and shoot. Go in the garden and shoot. Lie on the grass and shoot. Point in the distance and shoot. You'll learn a lot through practise with it.
There have been a few other threads in the past on Christenings as well. It might be worth running it through the search option to see if anything else has come up before that you think is handy. You might just get some help from someone who's not online tonight as well.
Hope I've been of some help for you and enjoy Sunday.
