Are children's books a good starting place for beginners?
There is one main difference between children's books and adult foreign textbooks:

Adult foreign textbooks are designed to give foreigners the best chance to communicate in simple everyday circumstances.

Children's books start with the very basics and require longer to get to the same place, however they provide you a much more solid base and a far wider range of knowledge.

So it depends on your aims. But if you are in for the long haul: I started with foreign language books and after a year was handed a set of Chinese primary school books. This gave me a huge benefit when going to the next level.