Why is devaluing your currency good for exports?
The problem lies with your assumptions. Practical world trade doesn't usually work that way. No one ever devalues their currency by half. 

This would be a practical assumption:

US buys toys from China - say x units. It also buy toys from Japan - say y units. 

Chinese toys cost xx, and Japanese toys cost yy.

Now, the US would start buying more from China (say x + a), by substituting Japanese toys ( now the US buys y - b) with Chinese toys. (The US would never import more without substituting because it won't import more than what it needs, as pointed out by someone else here)

If production is China had slowed down before devaluation, this would result in more production. More jobs, more income for employees. A devaluation is done to boost exports, resulting in more production, resulting in more employment, resulting in an overall increase in economic activity, whether or not it results in a direct increase in the income received through exports (although it usually does)