"Because the parents didn't have spend money on a lot of children, the parents could spend more on fewer children."
I think the ideal sentence should be:
"Since the parents decided not to raise many children, they could spend more on the few children they had."
I don't think this sentence is perfect. If you can make a better, please tell me.
But, how do you know if a sentence is bad? I can still understand what the first sentence is trying to say. Is it because it is too ambiguous? Most people would say the first sentence is weird, but what exactly is weird?