The higher the ratio of children to parents, the harder it is to raise them.
Literal
Parents [toward-に対する] children [possessive-の] ratio [subject-が] becomes-large [the-more-ば] [the-more-ほど] children [object-を] raising [topic-は] becomes-difficult.
~ば~ほど is the 'the more X, the more Y' pattern: 大きくなればなるほど = 'the larger it gets, the more...' This is one of the most useful comparative constructions in Japanese. に対する marks what the ratio is measured against.