What does In a spirit of a child mean?

In a spirit of a child, that is, you must not only have the heart of a child, but you must have the keen reason and common sense of an adult.

There are plenty of illusions, and there are many pitfalls. 

You need all your mental faculties. 

Be vigilant, accept everything as a child does, but examine and test everything, as an adult does, without prejudice and partiality.