I see where you're coming from -- but it can easily lead to bias against large families.
I come from such a family, we didn't starve and weren't medically neglected or anything, but there are a lot of people who might be inclined under a law like you propose to call my parents lawbreakers.
I don't equate having a large family with parents who have no means to adequately care for their children. Kids don't have to have everything that comes down the pike and some, I'm sure, are spoiled rotten.
Aside from that, we do want our children to "fit in" in some measure with their peers. It can be tough for many parents.
Designer jeans, the latest smart phone, Ivy League school, a high school graduation trip to Florida, etc. They really don't NEED any one of those things---they need to be taught a value system early on, including becoming educated. One of my cousins had 18 living children (two others died at birth). Other than two I've heard of who have had some problems, they've all done just fine. Were they wealthy? No! But they did their job.
I still believe those who can't support a child are morally responsible not to bring a baby into the world.