They might not want to talk to you either. I've been to a few Christenings and i certainly didn't talk to everyone there.
If you spot them, sit on the opposite side of the church, don't look their way whilst ensuring you are happy and smiley and then leave.
If you are cornered by them, again all happy and smiley and a quick "hello" is fine - no need to ask how they are or shake hands etc. As the great artist known as Miss Swift advises, "can't stop, won't stop moving" and shuffle your asses out of there.