Child development is a topic that can't be even close to adequately covered in a single blog post. Happily, many others have written at length about the topic, leaving you with lots to read and digest. Here are a few stand-outs. If you find others, please don't hesitate to post them in comments. It takes a village of social workers to help each social worker pass the ASWB exam!
Links like these (from one gov't website to another) can start to get you up to speed:
Check out these info-rich sites for lots and lots and lots and lots and lots more: