Ultimately if people feel comfortable that their children will grow up healthy and happy to adulthood it is widely accepted that the birthrate will tend to decr

Ultimately if people feel comfortable that their children will grow up healthy and happy to adulthood it is widely accepted that the birthrate will tend to decrease. So providing childcare. Any kind of reduced aid due to having (more) children would tend to punish people who have children. I think that you maybe able to find an ethical framework that matches your goals but that is just because there isn't a universal agreement on ethics. Also it's kind of impossible for people to understand the cost/benefits of having children because in general kids are amazing, unique and considered by many their primary source of happiness especially in later life. There are also familial pressures on young people to have children but I'm mostly familiar with life in a developed nation and I'm kind of assuming you are targeting the less developed countries with higher rates of poverty and birthrates. Either way I'm not a professional ethicist so probably not the advice you are seeking.