If you're installing Python3.5 on macOS, you don't need to follow the Automator instructions in Python for Kids any more. After installation, it's as simple as searching for IDLE in Spotlight (the magnifying glass in the top righthand side of the menu bar) and double-clicking on (usually) the top hit ("IDLE - Python 3.5").
You might still get an error message saying something like:
The version of Tcl/Tk (8.5.9) in use may be unstable. Visit http://www.python.org/download/mac/tcltk for current information
Tcl is a simple programming language and Tk is (sort of) a graphics library (more info here http://tldp.org/HOWTO/TclTk-HOWTO-3.html) -- the Tk graphics library is heavily used in later chapters of the book. If you happen to get the above error message, follow the instructions on the python.org website to install the correct version of ActiveTcl for the version of macOS you're running. This post may also be of help.