making child nodes accessible for iteration over subsections

This change just exports children so that users of the lib can iterate over subsections whose names may not be known ahead of time.

For example:

[service.HTTPD]
start_cmd = ...
stop_cmd = ...

[service.DB]
start_cmd = ...
stop_cmd = ...

Now I can write code to discover, start, and stop services from a TOML file without having to know the names of the services / service subsections, but instead I can iterate over all children of service.

This could probably be made much cleaner, but it was the minimum to do what I needed :-)