It is pretty darn rude just to get up and walk out. However, your kid pooing is a whole nother deal. Not only do you not want her to sit in it, or SQUISH in it, but I'm sure your pew neighbors don't want to sit next to it either.
It's also pretty darn rude for the hospitality minister to block your way!! If they're having a problem with people leaving early, maybe they need to re-evaluate the service!