So they lived broke all their life, yet they decided they could afford to have kids (which is probably one of the more expensive decisions a person can make)... and you say they are not over-spenders?
Nobody thinks they are going to be poor their entire lives.
This quote comes to mind:
Socialism never took root in America because the poor see themselves not as an exploited proletariat but as temporarily embarrassed millionaires. - Ronald Wright
Poor people have children because they want someone to take care of them when they get old -- doesn't seem like too much to ask.
Having said that, Catholic cultures in large do produce entirely too many offspring. They are afraid some of them are going to die along the way - and they usually do - but they still manage to overcompensate. Must be the whole anti-abortion, no condoms thing.