They never had 7 kids before. How would they know they couldn't handle it? In life, sometimes you just have to go for it.
Would they have all arrived at the same time? Common sense dictates that they would plenty of warning with the first ones, how problematic more would be. Common sense dictates that they would each need looking after, clothing, housing and feeding. Perhaps there was a lack of common sense, or more likely - they thought the state would fund them?