replace the if property template statement with :
if($property == 'template') {
if($this->childtheme()){
switch_theme( $this->template(), $this->childtheme() );
}else{
switch_theme( $this->template(), $this->template() );
}
}
add to the function list :
function childtheme() {
return $this->domain['childtheme'];
}
and then in your config add childtheme in the end
array(
'domain' => 'www.sitedomain.com',
'siteurl' => 'https://www.siteurl.com',
'home' => 'https://www.homeurl.com',
'blogname' => 'blog name',
'template' => 'maintheme',
'childtheme' => 'my_child_theme'
)