Poorest would be best. It'll help you put emphasize on what you're trying to say. Poor is very general, when poorest means you want to help the people are more poor than the general poor people.

The meaning is different, but you can use either. "Poor" children means all who are poor. That might be, say, 50% of the children. "Poorest" children means the ones who are very poor. It's a sub group of the poor children, so say 5% of the children who are poor.