I tend to disagree with Kiat, because children often create extra demand for products and services that would greatly contribute to the national GDP. Once China opened up to the West in the 70s, food supplies won't really be much of a problem, so starvation shouldn't be much of a factor to economic growth.
And I would think that even without the one child policy, Chinese population growth would have slowed down. Generally, developed areas tend to significantly slow down in population growth, so while the total population would be definitely higher, it certainly won't be "twice" it is now.
It probably will drive up food prices, but it still would be very gradually. At most, the current rises in food prices would simply have happened a decade or so earlier. It's not as if hundreds of millions of children suddenly disappeared after the implementation of the one child policy, and rises in demand probably can be met more or less. The general trend of the TFR was going down in china already (5 per women in the 50s to 3 by the 70s). Even after the implementation of the one child policy, it only went down to 2 per parent by the 90s. In all, the changes from OTL would have been gradual enough that changes could be accommodated by the world.That would certainly drive up the worlds food prices. They would also need to produce enough trade goods to afford these imports. And if they are stugeling to feed themselves will they have enough resources for "development"?
Actually, the curious thing about the one child policy is that, in rural areas (where most of China's population resides) it is largely ignored. Many children are actually hidden, or said to be orphans adopted by the village, etc. when census members and other government officials stop by.
While of course this isn't the universal situation, it is a far from uncommon practice amongst villagers.
One of the unforseen results of the one child policy has been the emergence of large numbers of 'spoilt brats' - particularly amongst the middle classes. These have begun to be a social problem and are indeed causing a change in society itself. Some have commented that in many cases, the child effectively has become the centre of family worship, replacing the elderly and ancestors in this role. Middle class Chinese youth has become self-centred and grasping... pretty much like western teens.
Parents tend to spend a considerable portion of their resources on their children, this amount doesn't vary much, it's just shared between the children there. The more children, the less each individual child has lavished on it. The traditional Chinese family tends to be more socialist in operation because of this.
The point of this post is that at some point these children will become 'the system' as they reach their 40's and their outlook on life will result in a change in the way China looks at itself and the world. If there was no single child policy, I'd expect this process to be less dynamic and China to remain pretty much as it is today.