$flat = { 
0 =>   { 'parents'  => undef, 'children' => [2,3,5] },
2 =>   { 'parents'  => [0], 'children' => [20,23] },
3 =>   { 'parents'  => [0], 'children' => [23,30,35] },
5 =>   { 'parents'  => [0],  'children' => [35,50] },
20 =>  { 'parents'  => [2], 'children' => [200,223] },
23 =>  { 'parents'  => [2,3], 'children' => [223,230,233] },
30 =>  { 'parents'  => [3], 'children' => [233,300,335] },
35 =>  { 'parents'  => [3,5], 'children' => [335,350] },
50 =>  { 'parents'  => [5], 'children' => [350,500] },
200 => { 'parents'  => [20], 'children' => undef },
223 => { 'parents'  => [20,23], 'children' => undef },
230 => { 'parents'  => [23],  'children' => undef },
233 => { 'parents'  => [23,30], 'children' => undef },
300 => { 'parents'  => [30],   'children' => undef },
335 => { 'parents'  => [30,35], 'children' => undef },
350 => { 'parents'  => [35,50], 'children' => undef },
500 => { 'parents'  => [50],    'children' => undef },
};

##</code><code>##

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