A one child policy definitely has advantages, such as less population growth and multiple resources and a greater chance for survival for the child. However, this does not justify a one child policy, since only children grow up with many problems, feelings of entitlement and the loss of companionship from siblings.
The most obvious benefit is that it can help with overpopulation. We have 7 billion people on this planet, and if we are unwise about our resources, it can be a spell for disaster.
It can also improve quality of life by allowing us to concentrate our resources. It can also raise some ethical options we wouldn't have previously considered. If we are only allowed to have one BIOLOGICAL child, but we are allowed to ADOPT to our heart's content, perhaps people would be more likely to consider adoption/fostering. This in turn, can really benefit the community. Currently the foster system is overcrowded and of low quality. The children are kicked out when they are 18, often to repeat the same cycle of poverty/abuse. If we could give these kids wonderful homes, perhaps we could help curb the cycle of poverty, abuse, and crime.
However, in practice the implication of this policy is quite horrifying. It all comes down to the fact that the policy is law, but our reproductive systems don't listen to the law. We can use contraceptive, but when those aren't effective, is it ethical to stick by the policy, to force sterilization or abortion on women?
Of course there are no advantages. What a stupid question. Government violence begets suffering and violence.
What if you have twins? What if you want more than 1 child, or are good at nurturing children? Isn't having options, better? What if you find practical or "religious objections" to "birth control"?
Birthing babies is just as natural and proper as breathing or the heart beating. Each and every human life is of immense value and sacred, so large families should be advocated worldwide, regardless or trendy or stupid "overpopulation" concerns.