Hello,
Regrettably, this can not be specified in the 
Path property of the web part since only one path expression can be defined.
However, it can be achieved by specifying multiple definitions of allowed content in index settings [
Site manager -> Administration -> Smart Search -> edit index -> Index]:
DOCUMENT TYPES:        PATH:          INDEX TYPE:
<document types>      /Section1           Allowed
<document types>      /Section1/%       Allowed
In other word you need to specify two definitions, one for the parent and one for child documents.
Best redards,
Michal Legen