I get confused between christenings and baptisms...is one religious and the other isn't, or are both religious, just from different religions?
For my little cousin's baptism, I gave her a nice card and a children's bible that was nicely illustrated and special. From my perspective, the baptism is done to bring a child into the parents religion, and any gift that aids in the religious training of the child is appropriate.
I think expecting money is tacky, personally. Is the child a boy or a girl? If you are open to buying things, maybe a necklace or bracelet for a girl? I favor giving gifts that are sentimental but also small and easily packed and stored. All I can think of for a boy is a pocketwatch, but that's kinda lame, I guess.
If you feel you must give cash, maybe find a cool picture frame and find a nice poem to go inside of the frame. Put a few crisp bills behind the poem or quote, and close it all up. I've done this for weddings; it satisfies my need to give something personal and useful.