First off, if you and your wife want to have another child, and one of you wants to stay home with the baby, and you're willing to sacrifice income to do it, then go for it. Unless money is your number 1 priority, then there are always going to be things that are more important to you, and there's nothing wrong with that. Once you've made that decision to give up income in order to invest more time/resources into something else, if there are things built into the tax code that will help you support your family, whether they're deductions, exemptions, or credits, go ahead and take advantage of them. You're not quitting your job so you can go on welfare as a form of paid vacation, or defrauding the state, you're just saying that money is not your top priority.
Once you make that decision, however, I'd say that it could be unethical if you attempt to implement a sort of "cap" on your income. Work within your principles and priorities, and don't do things that are going to violate them in order to up your salary, but if the chance to increase income comes along, and it's compatible with your way of life, take it. That may ultimately result in not getting the same tax credits, or it may not. That, for me at least, is the only real quandary here. I've known people who intentionally worked jobs that paid below a certain threshold so they could maximize government benefits (tax credits, TANF, SNAP, etc.), which makes those resources harder to get for people who really need them. IMHO, that's gaming the system. Taking a pay cut to take advantage of tax loopholes and credits is not cool. Taking a paycut so you can do what you love is awesome... and if the tax credits and up in your favor, so much the better.