I would wear something that a person would wear to church -- dress or skirt and blouse and some nice shoes. (For me, I always wear heels to church). Or, if you will be standing for awhile, something with a lower heel may be more comfortable.
If it's going to be in a church, I wouldn't wear a dress that is too revealing just out of respect for the church.
Then, after the Christening is done, you could change into whatever you wanted to.
