R.D.
A card would be perfect, and if possible, maybe a savings bond for the child which will mature and grow interest. I always give little personalized bible's as christening gifts, with a handwritten note to the baby inside.
In regards to the donation for the church, I'm not sure that's right. It's my understanding that the parents of the child being christened are supposed to make the donation, sounds like they're trying to take some of the bill off themselves, which sort of makes sense since they're paying for the 'party' afterwards, but still pretty tacky.