Select Code to Download

$flat = { 
0 =>   { 'parents'  => undef, 'children' => [2,3,5] },
...
350 => { 'parents'  => [35,50], 'children' => undef },
500 => { 'parents'  => [50],    'children' => undef },
};

$child = 2;
$gks   = @{$flat->{$flat->{$child}{children}[0]}{children}};
print join( ', ',  @$gks ), "\n";