Human beings, left to themselves, have imagined God in all sorts of shapes; but - although there were one or two instances, in Ancient Greece and Ancient Egypt, of gods being pictured as boys - it took Christianity to introduce the world to the idea of God in the form of a baby: in the form of complete dependence and fragility, without power or control. If you stop to think about it, it is still shocking. And it is also deeply challenging.
God chose to show himself to us in a complete human life, telling us that every stage in human existence, from conception to maturity and even death, was in principle capable of telling us something about God. (Rowan Williams, from his Christmas message.)